> only build PRs that have been tagged "documentation"

My preference would be to have a "preview-with-netlify" label that could be manually added to use the netlify preview on a case-by-case basis. There are a substantial number of trivial PRs (such as typo fixes and similar minor changes) that are automatically labelled as "documentation" which wouldn't particularly benefit from the netlify preview and would needlessly occupy part of the limit.

Personally, I feel that it most largely benefits more significant documentation changes that involve modification of existing markup or the addition of new markup, rather than all documentation PRs. So a manually added label to trigger it makes the most sense to me. This would be very similar to the recently added "test-with-buildbots" label.