Builtin docs (impl blocks, navigation table, link to Godot)

[Bromeon]

Changes:

• Vector types now have fewer useless impl blocks (e.g. separating constants).
• The remaining impls are named and anchor-linked from struct docs.
• For vectors, matrices, bounding boxes and strings, there is a new table with links to the other entries in the same category.
• Every builtin type (except packed arrays) now has a link to Godot doc page.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-821