Refactor: add BuildMode::Skip

[clarfonthey]

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Are you doing the PR on the next branch?

Note: this is rebased on top of #2418, and that should be merged first.

This is an attempt at incorporating the refactoring done in #2388 without changing behaviour. It helps pave a path toward render = false for pages while making things a bit more consistent for the existing render = false for sections and taxonomies.

Effectively, render = false will still add things to the library, but their pages and aliases won't be rendered. We could potentially convert having aliases for something render = false to a hard error, but at least for now, it makes no sense to create links to something that isn't actually rendered.

[clarfonthey]

Actually, going to close this first since I realise it needs a bit more work. After looking more into how the code works, get_page and get_section don't even go through SITE_CONTENT, so, we probably shouldn't rely on using BuildMode and instead just not running the write_content method.

This was the original justification for adding BuildMode::SKIP (since it bypasses SITE_CONTENT) but after going through it more, that's probably not the right choice.