Add a Testing API reference (plone.app.testing + plone.testing)

[jensens]

Builds on #2094 (the backend testing chapter). Targets backend-testing-docs; merge #2094 first. GitHub will retarget this to 6.0 once #2094 lands.

What

Adds an autodoc-generated API reference and a deeper layer-model explanation to the developer-guide/testing chapter, plus two how-tos. It covers both testing packages as one toolkit, because a test author does not care whether a symbol lives in plone.app.testing or plone.testing.

Page
testing-api-reference.md	Organized by task: layer model, pre-built layers & fixtures, browser & HTTP, helpers, sandboxing primitives, constants. Symbols documented at their canonical location (e.g. pushGlobalRegistry under plone.testing.zca, with a re-export note), from both packages.
how-testing-layers-work.md	Deepened with the fixture/lifecycle split.
install-add-ons-in-tests.md, drive-the-test-browser.md	Two how-tos absorbing the test-specific patterns from plone.app.testing's README.

How the reference is built

sphinx-autodoc2, which analyses the sources statically. So:

• No Plone is installed in the docs build. sphinx.ext.autodoc would need to import plone.app.testing, which pulls in ~226 packages (all of Plone). autodoc2 needs neither. Both packages are added as source-only submodules.
• It coexists with the existing sphinx.ext.autodoc (used by plone.api).
• The plone.testing submodule pins master; the Layer class docstring there is currently a one-liner. Expand the Layer base class docstring plone.testing#134 enriches it, after which a submodule bump renders it in full. The reference is complete either way; that entry just gets richer.

Notes

• Browser is a re-export from a private module (plone.testing._z2_testbrowser), which autodoc2 cannot resolve, so it is documented as a short prose entry.
• Legacy ZServer (PLONE_ZSERVER, PLONE_FTP_SERVER) is noted, not documented — Plone 6 runs on WSGI.
• Vale is clean on the new pages; the vocabulary additions it needed are included and sorted.

🤖 Generated with Claude Code

[gforcada]

Very nice! 🌟 I have only a few suggestions for improvements rather than blockers 👍🏾

[jensens]

Thanks @gforcada! Addressed in the last commit:

• requirements.txt — sorted sphinx-autodoc2 into place.
• conf.py comment — good point about it outdating. Made the comments generic ("static API docs from source, no package import needed") instead of naming a specific consumer, so they stay true no matter which packages use autodoc2.
• "which both packages?" — now names them: "across plone.app.testing and plone.testing".
• pytest note — made it a {tip}.
• Blicca — done, and thanks for the nudge: "Blicca, the frontend formerly called Classic UI", with a link to the Volto testing docs for the other side. (The rest of the docs still say "Classic UI", so this page is slightly ahead — happy to align once the wider rename lands.)

Two where I did something a bit different, curious what you think:

• defaultBases vs bases — they aren't quite the same purpose, so rather than drop one I clarified when each applies: you set defaultBases on a reusable layer class, and pass bases only when instantiating a layer directly to combine existing ones (the exception, not the rule). That should answer the "which one" question without hiding the override.
• The intro "documented at its canonical location" note — I kept it as prose rather than a third admonition. The page already has two (the legacy-ZServer warning and the re-export note), and the Plone docs style guide asks for admonitions sparingly, so I didn't want to spend a third on orientation text. Easy to switch if you feel strongly.