chore: reorganize docs folder with numbered prefixes

[moul]

Continues #1801

TODO:

• rename files
• setup linters checking for 404 links
• fix 404 errors

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[moul]

Current state:

See https://github.com/golang/go/tree/master/doc/next

[moul]

@leohhhn could you help me continue this PR, please?

[leohhhn]

Hey @moul, I need push permissions to your branch. I've renamed the files as asked, but cannot push :)

[leohhhn]

To not wait for @moul, I've created my own branch for this: #2125. Closing this PR.