watchman: update main page

[ktz-dev]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.

[ktz-dev]

On request, closed the PRs for sub-command pages and instead added some commands to the main page.

Also, the watchman watch command works for now but will soon be deprecated in favour of the watchman watch-project command. See the docs.

Deprecated starting in version 3.1. We recommend that clients adopt the watch-project command.