[Sync-EN] Convert extensions.ent to the XML entity format

[lacatoire]

Syncs the extension categorization entities with php/doc-en#5697, plus the two docbookcs.xml commits that sit on the same file, which the revision check pulls in.

php/doc-en#5697:

• extensions.ent (DTD syntax) and entities/entities.ent are replaced by a single entities/entities.extcat.ent in the translatable XML entity format, matching the upstream layout.
• Only the entities the appendix actually uses are kept: intro, alphabetical, the four membership ones and the three state ones. The extcat.purpose.* entities are dropped, as upstream: nothing references them any more, in appendices/extensions.xml or elsewhere.
• The Russian wording is carried over from the two old files, with the paragraphs as simpara and the intro, membership and state texts aligned with the current English ones.

docbookcs.xml:

• The entity configuration collapses to ../doc-base/temp/doctype.dtd, replacing the per-file list, which also removes the now dangling extensions.ent entry.
• WhitespaceSniff is replaced by MixedIndentationSniff and TrailingWhitespaceSniff.

Fixes: #1572