Docs: document search_show_subprojects_filter addon option

[Copilot]

The search_show_subprojects_filter option (added in #12684) was never documented, leaving no public reference for how to disable the subprojects filter in the search modal.

Changes

• docs/user/server-side-search/index.rst — Expanded the "Search across subprojects" feature entry to note that the search modal includes a subprojects filter and that it can be disabled via Settings > Addons > Search > Show subprojects filter in search modal.

• docs/user/addons.rst — Added a "Disabling the subprojects filter in search" subsection under "Configuring Read the Docs Addons" explaining the option, its use case (e.g. avoiding cross-contamination between a project and its dev/staging subprojects), and where to find it in the UI.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Docs: document about disabling searching on subprojects</issue_title>
<issue_description>## What's the problem this feature will solve?

We have an option for this in the addons search_show_subprojects_filter, it was added in #12684, but isn't documented.
</issue_description>

<agent_instructions>Document this in server-side-search/index.rst, and maybe mention it in addons.rst.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Docs: document about disabling searching on subprojects #12801

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[read-the-docs-community]

Documentation build overview

📚 docs | 🛠️ Build #31480819 | 📁 Comparing 34784e5 against latest (4725bf8)

🔍 Preview build

Show files changed (2 files in total): 📝 2 modified | ➕ 0 added | ➖ 0 deleted

File	Status
addons.html	📝 modified
server-side-search/index.html	📝 modified