Server should build project before generating preview, if necessary

[alcarney]

Expected behavior

This used to be the case in 0.91.0 of the extension, then it was removed because of #660 and similar issues.
Now however, it should be safe to bring this back and is a common point of friction (#763, #778, #699 (comment))

Actual behavior

The server automatically trigger a Sphinx build of a project (if necessary) before opening the preview.

Log output

No response

(Optional) Settings from conf.py

No response