Improve the appearance of crates on `crates.io` (#5243)

## Context

Currently, many crates have no readme files, even though they are public
and available on crates.io.
Others have just a couple of words that does not look super presentable.

Even though probably nobody starts a journey with `polkadot-sdk` or its
documentation with a readme of a random low-level crate, I think it
would look more mature to have a little better readmes there.

So, in an attempt to improve [the
aesthetics](https://github.com/paritytech/eng-automation/issues/10) of
`polkadot-sdk`, I propose a set of consistent, branded, better-looking
readmes for all published crates.

## What's inside

- ~~New readme files for published crates.~~
- A python script to generate new readmes, for the crates that have
none.
  - It will skip crates that do have a readme, and private crates.
- Added a new image asset to the repo - logo with a background.
- The main readme of the repo uses a [nice
trick](https://github.com/paritytech/polkadot-sdk/blob/ce6938ae/README.md?plain=1#L4-L5)
to accompany both light and dark modes - but that doesn't work on
`crates.io` so a single logo with a background is needed.

## Example

### Current

![Screenshot 2024-08-04 at 16 13
36](https://github.com/user-attachments/assets/3ae0881d-0f40-4614-9d2c-3c95029f0820)

### Changed

![Screenshot 2024-08-04 at 16 12
28](https://github.com/user-attachments/assets/fa7eadc8-aec8-4f77-84d9-54d63ce189cd)

.github/scripts/generate-readmes.py

+#!/usr/bin/env python3
+
+"""
+A script to generate READMEs for all public crates,
+if they do not already have one.
+
+It relies on functions from the `check-workspace.py` script.
+
+The resulting README is based on a template defined below,
+and includes the crate name, description, license,
+and optionally - the SDK release version.
+
+# Example
+
+```sh
+python3 -m pip install toml
+.github/scripts/generate-readmes.py . --sdk-version 1.15.0
+```
+"""
+
+import os
+import toml
+import importlib
+import argparse
+
+check_workspace = importlib.import_module("check-workspace")
+
+README_TEMPLATE = """<div align="center">
+
+<img src="https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/docs/images/Polkadot_Logo_Horizontal_Pink_BlackOnWhite.png" alt="Polkadot logo" width="200">
+
+# {name}
+
+This crate is part of the [Polkadot SDK](https://github.com/paritytech/polkadot-sdk/).
+
+</div>
+
+## Description
+
+{description}
+
+## Additional Resources
+
+In order to learn about Polkadot SDK, head over to the [Polkadot SDK Developer Documentation](https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/index.html).
+
+To learn about Polkadot, visit [polkadot.com](https://polkadot.com/).
+
+## License
+
+This crate is licensed with {license}.
+"""
+
+VERSION_TEMPLATE = """
+## Version
+
+This version of `{name}` is associated with Polkadot {sdk_version} release.
+"""
+
+
+def generate_readme(member, *, workspace_dir, workspace_license, sdk_version):
+    print(f"Loading manifest for: {member}")
+    manifest = toml.load(os.path.join(workspace_dir, member, "Cargo.toml"))
+    if manifest["package"].get("publish", True) == False:
+        print(f"⏩ Skipping un-published crate: {member}")
+        return
+    if os.path.exists(os.path.join(workspace_dir, member, "README.md")):
+        print(f"⏩ Skipping crate with an existing readme: {member}")
+        return
+    print(f"📝 Generating README for: {member}")
+
+    license = manifest["package"]["license"]
+    if isinstance(license, dict):
+        if not license.get("workspace", False):
+            print(
+                f"❌ License for {member} is unexpectedly declared as workspace=false."
+            )
+            # Skipping this crate as it is not clear what license it should use.
+            return
+        license = workspace_license
+
+    name = manifest["package"]["name"]
+    description = manifest["package"]["description"]
+    description = description + "." if not description.endswith(".") else description
+
+    filled_readme = README_TEMPLATE.format(
+        name=name, description=description, license=license
+    )
+
+    if sdk_version:
+        filled_readme += VERSION_TEMPLATE.format(name=name, sdk_version=sdk_version)
+
+    with open(os.path.join(workspace_dir, member, "README.md"), "w") as new_readme:
+        new_readme.write(filled_readme)
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(
+        description="Generate readmes for published crates."
+    )
+
+    parser.add_argument(
+        "workspace_dir",
+        help="The directory to check",
+        metavar="workspace_dir",
+        type=str,
+        nargs=1,
+    )
+    parser.add_argument(
+        "--sdk-version",
+        help="Optional SDK release version",
+        metavar="sdk_version",
+        type=str,
+        nargs=1,
+        required=False,
+    )
+
+    args = parser.parse_args()
+    return (args.workspace_dir[0], args.sdk_version[0] if args.sdk_version else None)
+
+
+def main():
+    (workspace_dir, sdk_version) = parse_args()
+    root_manifest = toml.load(os.path.join(workspace_dir, "Cargo.toml"))
+    workspace_license = root_manifest["workspace"]["package"]["license"]
+    members = check_workspace.get_members(workspace_dir, [])
+    for member in members:
+        generate_readme(
+            member,
+            workspace_dir=workspace_dir,
+            workspace_license=workspace_license,
+            sdk_version=sdk_version,
+        )
+
+
+if __name__ == "__main__":
+    main()