ci(docs): scope pull_request triggers to integration branches

[Mpdreamz]

Scopes docs-build and docs-preview-cleanup pull_request* workflows to integration branches (assembler.yml).

Related: https://github.com/elastic/docs-eng-team/issues/569

[mergify]

This pull request does not have a backport label. Could you fix it @Mpdreamz? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[v1v]

what's the reason no support for release branches as done at elastic/beats#50373, those repositoires are enrolled to the Unified Release