docs: fix broken links

[besaleli]

• Inlines CONTRIBUTING.md and CODE_OF_CONDUCT.md into docs
• Fixes broken links
• centered badges on docs homepage
• the diagram was a little hard to see in dark mode, so I changed the theme
• add footnote about glibc

Does not fix the following, so leaving as an FYI:

• Doc file 'cookbooks/token-classification-ner.md' contains a link 'sequence-classification-sentiment.md', but the target 'cookbooks/sequence-classification-sentiment.md' is not found among documentation files.
• Doc file 'cookbooks/token-classification-ner.md' contains a link 'embeddings-similarity.md', but the target 'cookbooks/embeddings-similarity.md' is not found among documentation files.
• Doc file 'cookbooks/token-classification-ner.md' contains a link '../index.md#-installation', but the doc 'index.md' does not contain an anchor '#-installation'.

Test locally by running make docs and going to http://localhost:8000. Let me know if you're happy with the changes!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

CodSpeed Performance Report

Merging #135 will not alter performance

_{Comparing docs/fix-broken-links (99e106a) with main (2be28bb)}

Summary

✅ 20 untouched
⏩ 20 skipped¹

Footnotes

1. 20 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[angpt]

Fixed broken links in the cookbook with this PR.