doc: move octezdoc.sh from doc_publish to normal doc build

What

Fixes a failure in CI job publish:documentation that appeared after merging !16280 (merged).

Why

The CI job publish:documentation failed because it had not all the needed artifacts, in particular those of CI jobs manuals and docgen.

How

Instead of adding those artifacts (which would be difficult to test before merging), we systematically build octezdoc.txt along the normal doc build. It only adds about 30s to the build, and this change can be tested in the regular CI on the MR branch.

Manually testing the MR

Check in the CI that the doc build_all is green. Check in its log that sphinx war ran a second time with the text builder, and check in the artifacts that they contain file "_build/octezdoc.txt".

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR