Update documentation pages for Symbolic Macros, Finalizers, and Macro-Aware Visibility

[brandjon]

Kinds of documentation to update:

• [ ] Concept pages for macros
• [ ] API docs (macro() callable, etc.)
• [ ] Concept pages for visibility
• [x] Executable example in examples repo

[brandjon]

@bazel-io flag 8.0.0

[iancha1992]

@bazel-io fork 8.0.0

[Wyverald]

What's the status of this? Can we get docs done by 25 Nov?

[tetromino]

@Wyverald - it's in progress; @brandjon is updating visibility docs, and I have in review updates for docs for the macro() built-in and the symbolic macros introduction doc. We should be able to get it done by 25.

[brandjon]

@wyverald There's three CLs to be cherrypicked imminently that'll close off this issue.