Code Conventions link in CONTRIBUTING.md leads to 404

[Keymasterer44]

pyrefly/CONTRIBUTING.md

Line 79 in 1896ffe

[Buck2 coding conventions](https://github.com/facebook/buck2/blob/main/HACKING.md#coding-conventions),

The URL in the above linked line leads to a GitHub 404 page for me stating "The main branch of buck2 does not contain the path HACKING.md.".

[yangdanny97]

buck removed it in facebook/buck2@89f73f3

maybe we can link to this instead: https://github.com/facebook/buck2/blob/main/docs/developers/basics.md

[GHX5T-SOL]

I'll send a narrow docs PR updating the Buck2 coding-conventions link to the current \ page suggested above.

[GHX5T-SOL]

I'll send a narrow docs PR updating the Buck2 coding-conventions link to the current docs/developers/basics.md page suggested above.