uinput: tidy up the README

The main changes are:

* Describe how multiple commands are passed to `uinput`
* Split details about waiting after device registration into its own
  section
* Add details on the JSON extensions `uinput` uses
* Change example code blocks to JSON5 syntax to better accommodate the
  JSON extensions
* Use absolute links to Android Codesearch instead of relative ones
* Fix various Markdown issues and improve formatting

Bug: 280604085
Test: check rendering in Google Markdown
Change-Id: Ic6d5f3e9d1415fcd9425b1afc767574a65556578