Move all database guides under the database directory

Following the completion of !30929 (merged), we should move all the database guides under the doc/development/database/ directory.

Context from the related the discussion:

[..] I think we should remove the links from development/README.md and add them to database/index.md instead, as you did. From development/README.md, we link to database/index.md so everyone that lands there will know where the docs are.

Then, I think we should (incrementally) move all database dev docs under database/*, to get to a better structure. The reason I say that is that having all under the same dir allows navigating among them through the breadcrumbs.

[..]

That also works best for SEO, since crawlers look for the file tree. And that will be also important for automating the docs left-side nav, as we plan to do in the future.

While moving all the files, we should also make sure that any links to those guides are updated accordingly.