docs(parachain): design for bitfield distribution

[haikoschol]

Changes

Add the design document for the bitfield distribution subsystem.

Issues

#4232

[haikoschol]

Kept as draft until we decide where under docs we want to put design docs.

[P1sar]

can you please put this in to design-proposals folder?

[haikoschol]

can you please put this in to design-proposals folder?

According to the meeting notes, we decided on /docs/docs/design/parachain_protocol. Because it is already used in the docs folder (and because it looks better :) I've used kebab-case and put it in /docs/docs/design/parachain-protocol.

[kishansagathiya]

You would want to put a link to this doc in the navigation bar. you would need to modify mkdocs.yml for that.

[timwu20]

LGTM! Great work @haikoschol 👍

[haikoschol]

I've changed the branch this is based off from feat/parachain to development.

As discussed on #4349, we decided to add design documents to development to have them show up on https://gossamer.chainsafe.io/. The name of the feature branch is misleading now, but I figured it's the lesser evil over opening a new PR instead.

@edwardmack If you want to do the same on #4353, this is what I did:

1. checked out my feature branch locally
2. copied bitfield-distribution.md somewhere outside the repo
3. checked out development
4. renamed my feature branch (git branch -m haiko/feat/parachain/bitfield-distribution-design haiko/feat/parachain/bak-bitfield-distribution-design)
5. created a new feature branch with the previous name
6. copied bitfield-distribution.md to docs/docs/design and committed the added file
7. edited availability-subsystems.md to link to bitfield-distribution.md and committed the change
8. force pushed the new feature branch to github