A follow-up to https://github.com/paritytech/polkadot-sdk/pull/5447

Instead of having an option to select a PR template (like is the case
with issues), this change will make the one PR template we have the
default, it will show up for every new PR.

Also updated one link so it works properly in the PR body.

Hello
I found several spelling errors.
Br, Elias.

Uses custom metadata to exclude chain-specific crates.  
The only concern is that devs who want to use chain-specific crates,
still need to select matching versions numbers. Could possibly be
addresses with chain-specific umbrella crates, but currently it should
be possible to use [psvm](https://github.com/paritytech/psvm).

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

An attempt to improve [the
docs](https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/index.html)
by applying various corrections:

- grammar/stylistics,
- formatting,
- broken links,
- broken markdown table,
- outdated vscode setting name,
- typos,
- consistency,
- etc.

Part of https://github.com/paritytech/eng-automation/issues/10

Co-authored-by: Bastian Köcher <git@kchr.de>

cc https://github.com/paritytech/ci_cd/issues/974

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Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <info@kchr.de>

Updating the prdoc doc file to be a bit more useful for new contributors
and adding a SemVer section.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

cc https://github.com/paritytech/ci_cd/issues/943

A brief explanation of the Deprecation Checklist is added to the
Contributing notes with a link to it

This PR is a continuation of
https://github.com/paritytech/polkadot-sdk/pull/2102 and part of an
initiative started here https://hackmd.io/@romanp/rJ318ZCEp
What has been done:
- The content under `docs/*` (with the exception of `docs/mermaid`) has
been moved to `docs/contributor/`
- Developer Hub has been renamed to Polkadot SDK Docs, and the crate has
been renamed from `developer-hub` to `polkadot-sdk-docs`
- The content under `developer-hub/*` has been moved to `docs/sdk`

---
Original PR https://github.com/paritytech/polkadot-sdk/pull/2565, it has
been close due to too many rebase conflicts

---------

Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: Chevdor <chevdor@users.noreply.github.com>
Co-authored-by: Egor_P <egor@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>