docs(private packages): rewrite pip-compile section

[HonkingGoose]

Changes

• Rewrite the pip-compile section:
  • Use Material for MkDocs style admonition, bump to caution level styling
  • Use lists
  • Add headings to split information into clearer chunks
  • Explain who needs the info in the self-hosted section
  • Split long sentences
  • Use the word must when users must do something

Context

Follow up after:

• docs(pip-compile): Add note re proper usage of index-url #29421

I went looking into this file because the admonition the author added is in the GitHub Markdown style, which Material for MkDocs does not render properly. But then I found other bits, and it ballooned into this PR. 😄

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

@craigds can you review for correctness?

[renovate-release]

🎉 This PR is included in version 37.410.2 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.410.2

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