Improved cross-referencing in documentation

Makes some simple changes to the documentation as per #5405 to allow easier cross-referencing of members:

* `[page:ClassName title]` now generates `<a id="title" ...>` tags
* `[member:.memberName title]` and `[member:Class.memberName title]` can be used to link to class members. This generates URLs of the form `docs/#section/category/ClassName/memberName`, which automatically scroll to the associated anchor.
* `[example:example_name title]` generates `<a href="../../../examples/#example_name" ...>`

I've added some examples of this new syntax to the page for `BufferGeometry`. All other pages should work exactly as before. Should be helpful for #5411.