Docs: remove doc markdown from this repo; earthstar-docs is the new source of truth

[cinnamon-bun]

Now that we have https://github.com/earthstar-project/earthstar-docs , empty out the docs/*.md files from this repo.

Change the content of each one to be just a note saying it's moved, with a link to

• the docs website: https://earthstar-docs.netlify.app/docs/
• and the github folder where the markdown files are, https://github.com/earthstar-project/earthstar-docs/tree/main/docs

Look around the rest of this repo for more links to docs and update them (in README, CONTRIBUTING, issues, PRs, releases?)

[cinnamon-bun]

Docs in this repo are now just pointers to the earthstar-docs repo.

Still need to update the README links to point directly at earthstar-docs.

Also should migrate some other things to earthstar-docs:

• important conversations in issues that are referenced from the README
• updated invited code format and URL formats
• maybe a snapshot of the wiki about standard data formats used by apps