docs: fix DEVELOPMENT.md links in CONTRIBUTING

[l46983284-cpu]

Summary

The published contributing page (…/docs/contributing.html#improving-the-gnu-compatibility) turned relative DEVELOPMENT.md links into DEVELOPMENT.html URLs that 404.

This updates the GNU compatibility and code coverage links (and the other DEVELOPMENT.md references in CONTRIBUTING.md) to GitHub blob URLs so they resolve from the docs site.

Test plan

• Inspect rendered markdown links point at https://github.com/uutils/coreutils/blob/main/DEVELOPMENT.md…
• Confirm section on the published contributing page no longer 404s after merge/docs deploy

Fixes #7201

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[sylvestre]

sorry but i would like the documentation to point to the local file, not to a website

[l46983284-cpu]

Thanks @sylvestre — updated to local relative links (./DEVELOPMENT.md and section anchors) instead of GitHub blob/website URLs. Ready for another look.

[xtqqczze]

@l46983284-cpu The links are not broken, this PR and the linked issue can be closed:

https://uutils.org/coreutils/docs/DEVELOPMENT.html -> 200

[l46983284-cpu]

You're right — published DEVELOPMENT.html is 200 now; #7201 looks fixed on the docs site. Closing this PR. Leaving the issue for maintainers to close if they agree.