GRO-835 Serve/hot-reload local metabase files

[bpander]

GRO-835

Description

Adds METABASE_REPO_PATH environment variable to optionally view local mb repo files.

Also adds README.

How to test

1. cp .env-dist .env
2. Point METABASE_REPO_PATH at your local metabase repo
3. bun dev
4. Navigate to http://localhost:4321/docs/latest
5. Make changes to metabase repo docs md files, images, etc
6. Verify changes are reflected on dev server
7. Unset METABASE_REPO_PATH (comment it out or something) and restart dev server
8. Verify dev server serves what's in _docs and looks the same as before

Would also be good to test update-docs-base-bun.yml won't act funky with these changes, e.g. script/update_docs_for_branchname.clj calls script/docs which uses constructDocMetadata and reformatMarkdownUrls which were moved and changed to TS.

PR env: https://gro-835-hot-reload-local-metabase.dkkh35gceu2po.amplifyapp.com/docs/latest/

Demo

demo-short.mov

[linear]

GRO-835

[gantoreno]

🚢🚢🚢

One minor thing I found, won't block on approval but needed before merging is that the "Propose a change" widget link is now a) missing file extensions and b) trimming the filename in case it's README.md, which causes those links to always 404

For example, on /docs/latest, the link should be: https://github.com/metabase/metabase/blob/master/docs/README.md but instead it's https://github.com/metabase/metabase/blob/master/latest/

[bpander]

the "Propose a change" widget link is now a) missing file extensions and b) trimming the filename in case it's README.md, which causes those links to always 404

Thanks for catching that @gantoreno 🙏 - I did some late-stage cleanup and didn't test it thoroughly enough. Addressed in 1063df2. Computes the docPath using the doc's actual filePath. Added handling for the !METABASE_REPO_PATH branch too which is technically unreachable in its current state but will be reachable soon enough (when we nix script/docs).