chore: don't rebuild docs from scratch when using make docs-serve

[sloria]

Description

The docs take quite a long time to build from scratch (>1m on my old pre-Apple Silicon macbook air). After the initial build, they build quickly, so long as the _build dir isn't removed. I tend to kill then re-run sphinx-autobuild pretty often, so this ends up saving a lot of time

Closes

[sloria]

also, sorry for the barrage of tiny PRs 😬 some people are pretty opinionated about these little DX things, so i opted for independent PRs so they could be merged separately

[provinzkraut]

Mmh. Not sure this is ideal. There's a reason we do the clean and rebuild, because some things don't always rebuild correctly, so I'm a bit hesitant to merge this one. @JacobCoffee?

[sloria]

yes, there are rare cases where stale build files end up in docs/_build, but imo the small cost of having to run make docs-clean manually for those cases doesn't outweigh the much faster builds for the common case.

[sloria]

also note that make docs-serve is only used for local development, so having clean docs/_build isn't as critical. i left the make docs command alone, since it does get used in CI so having a clean build dir makes more sense there

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3957