pw_docgen: Add sphinxcontrib-mermaid

This allows for embedding mermaid diagrams in rst files. It uses
https://github.com/mgaitan/sphinxcontrib-mermaid

For example:

.. mermaid::

   sequenceDiagram
      participant Alice
      participant Bob
      Alice->John: Hello John, how are you?
      loop Healthcheck
          John->John: Fight against hypochondria
      end
      Note right of John: Rational thoughts <br/>prevail...
      John-->Alice: Great!
      John->Bob: How about you?
      Bob-->John: Jolly good!

Change-Id: I04ee218cfb6e3a6cfdd3f3e189a5a632d5628bee
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44021
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>