Add PHP manual cross-reference entities and update build configuration - #15

LGTM. A very extensive and yet very detailed change.

• Cleanup language-snippets.ent and other stale entities

After this manual builds, a very easy way to detect stale entities is to list all entities, and then: 1. checkout local clones; 2. programmatically erases or renames an entity; 3. try to build the manual. If the manual keeps building, then that entity is unused. It's a lot of local CPU burning, yes, but will produce the result.

• doc-base/scripts/broken.php behaves as if this is a translation repo

I think it's possible to change the test for $dnt to inspect the existence of manual.xml in the lang dir. I will try to make this change, after I commit the last PRs still open.

• The exception classes xi:include their inherited members from doc-en, those can't resolve here and render as placeholders

They are reported by configure.php, and probably will be replaced by new normal XML entities, copied from the respective targets, from the (assembled) main manual.