As discussed before we selected win_environment to the documentation,
and point to win_uri for a more advanced module.
If we want to make this the reference module, we have to get this one
absolutely right in every possible way.
This PR cleans up both win_environment and win_uri, and makes the
required changes to the windows module development section.
* Added initial developing module doc for Windows
* Added to dev index, made formatting changes and moved obsolete docs
* changes based on PR feedback
* some typos and extra information
* Minor updates
* minor wording clarifications
* remove references to "remote" Windows hosts
* update template/text to remove legacy `#WANT_JSON`/`#POWERSHELL_COMMON`
* use per-execution envvars for ANSIBLE_KEEP_REMOTE_FILES instead of stateful
* fix up minor sanity check issue
