Docs site: a page on main before its 10.x cherry-pick fails the 10.x build and stops publishing #420

Problem

site/build.sh builds both lines with main's site/toc.yml. A page that lands on main before its cherry-pick to release/10.x is in that menu but missing from the 10.x checkout. DocFX reports the missing menu target as a warning, and the build treats warnings as errors.

So from the first such page on:

• Every docs PR into main fails its Build (10.x) check.
• The live site stops updating. The publish job needs both builds, so neither line is republished until the cherry-pick lands.

CONTRIBUTING.md#the-documentation-site tells contributors to add a new page to site/toc.yml, which is exactly the step that triggers it. The walkthrough (#418) will be the first page to do so.

Found while wiring #418 into the site, before any page had triggered it.

Change

When site/build.sh builds a checkout other than its own, it removes the menu entries whose page doesn't exist in that checkout, and logs each one. When it builds its own checkout, a missing page is still an error, so main's menu is still checked in full. CONTRIBUTING says that a page on main only shows up in the 10.x menu once it's cherry-picked.

Done when

• With a main-only page in toc.yml, the 10.x build passes without that entry and logs it, and the 11.x build includes the page.
• A menu entry naming a page that doesn't exist on main still fails the 11.x build.