##  introduce AI patches (beta) documentation and update navigation

### Description

This PR adds documentation for the new **AI patches (beta)** feature in
OSS-Fuzz

### Detailed Changes

* **New documentation page:**
* Created `docs/AI patches (beta).md` to explain the benefits, scope
(C/C++ memory safety vulnerabilities), inner workings, quality assurance
(patch shepherds), sign-up/opt-out instructions, and feedback channels
for the AI patch program.
* **Bug-fixing guidance update:**
* Updated `docs/advanced-topics/bug_fixing_guidance.md` with a new
`AI-proposed patches` section to advise maintainers on how to prioritize
bugs that include an AI-proposed patch (noting they require less time to
fix and have passed quality checks).
* **Navigation and ordering adjustments:**
  * Assigned the new page a navigation order of `3`.
* Incremented `nav_order` by 1 for subsequent documentation sections to
preserve consistent ordering:
    * `docs/advanced-topics/advanced_topics.md` (moved from 3 to 4)
    * `docs/further-reading/further_reading.md` (moved from 4 to 5)
    * `docs/reference/reference.md` (moved from 6 to 7)
    * `docs/faq.md` (moved from 7 to 8)

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Co-authored-by: Alex Kilian <akilian@google.com>
Co-authored-by: Dustin Ingram <di@users.noreply.github.com>