Moves most of the documentation into a dedicated `docs/README` to make
it easier to reach important content in the top-level readme like
"getting involved".
There are some minor changes to the heading levels here and link
changes, but otherwise the content is the same. Don't go deep on the
getting started section, that's changing in #329
Mostly making sure that the script is robust to alternative working
directories, and some stylistic nits.
Following up on
https://github.com/astral-sh/ty/pull/65#discussion_r2077839514 — not to
be annoying, but using `git` to find the root _can_ be wrong here
```
❯ cd ruff
❯ uv run --only-dev ../scripts/update_schemastore.py
> /Users/zb/workspace/ty/scripts/update_schemastore.py(146)main()
-> breakpoint()
(Pdb) print(root)
/Users/zb/workspace/ty/ruff
```
