Remove acronyms entity file from doc-base

[jordikroon]

Suggested by @alfsb

Moving Acronyms from doc-base to doc-en. This allows Acronyms to be translated, as well as to more easily phase out unused acronyms.

This is a set of 3 PRs
doc-en: php/doc-en#5695
doc-base: #325 (you are here)
phd: php/phd#262

[lacatoire]

Nice move. Acronym expansions were one of the last reader facing strings stuck in English, and this puts them in the right place.

[alfsb]

This may conflict with PR 381, but as noted there, I may change PR 381 as the hand off is done first, and deletion of PhD functions is done after PhD side is validated and merged.

In fact, I ask for your testing of PhD side of PR 381. Comment there if there is any change of hand off file contents or format, as to facilitate this process.

[jordikroon]

Currently tests fail because doc-base now depends on doc-en. Once that's merged the entities should resolve for both doc-en and other languages.

This might conflict with 381, but if it does, it would be resolved fairly quickly as it's a 3 line removal that is covered in both this PR as well as in 381. Will give it a review tomorrow.