pw_watch: Add option to serve docs from a local webserver

Add options:
--serve-docs
--serve-docs-port
--serve-docs-path

Change-Id: Id339358709a75636d4e3a476280ad25ac100f9d1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/53340
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>