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)