Move GitBook {% hint %} conversion upstream into the guidebook repo (remove transitional staging workaround)

[sanchuanhehe]

Background

PR #7 (fixing #4) made GitBook {% hint style="..." %}...{% endhint %} callouts render correctly by rewriting them into MyST admonitions at build time in the staging tree, via tools/convert_gitbook.py (invoked from tools/prepare-source.sh).

This is a transitional workaround, deliberately chosen so we don't fork/modify editorial content in the guidebook submodule. It is not the intended long-term state.

Why it's temporary / the problem

• The source of truth (theopensourceway/guidebook) still contains GitBook-specific syntax that no current renderer (MyST/Sphinx) understands.
• The conversion only exists in this production build pipeline, so anyone consuming the guidebook directly still sees broken {% hint %} markers.
• It adds a build-time transform + maintenance surface (tools/convert_gitbook.py, an extra prepare-source.sh step) that has to be kept in sync with whatever hint styles appear upstream.

Proposed long-term fix

1. Convert {% hint style="..." %}...{% endhint %} to MyST admonitions directly in the guidebook repo source Markdown:
   • success → tip
   • info → note
   • warning → warning
   • danger → danger
2. Once upstream is converted, remove tools/convert_gitbook.py and its invocation in tools/prepare-source.sh.
3. Re-extract gettext catalogs and confirm zh_CN .po msgids are unchanged (the admonition body text is the translatable unit, so translations should carry over).

Acceptance criteria

• No {% hint %} / {% endhint %} syntax remains in the guidebook repo source.
• tools/convert_gitbook.py and the prepare-source.sh conversion step are removed.
• EN and zh_CN HTML + PDF still render the admonitions correctly.
• zh_CN catalogs remain current (git diff --exit-code -- locales passes in CI).

References

• Original bug: GitBook {% hint %} blocks not rendered — displayed as literal text #4
• Implementing PR: fix: guidebook rendering & zh_CN i18n issues (#1-#5) #7 (search for the "transitional" note)
• Affected source pages: growing-contributors/community-manager-self-care.md, attracting-users/communication-norms-in-open-source-software-projects.md