Improve environment variable handling

[felixfontein]

Improve environment variable handling:

• Use the correct Sphinx markup for environment variables (envvar role).
• Create an environment variable index.

Examples:

• Index: https://ansible.fontein.de/collections/environment_variables.html
• Enviornment variable in a plugin: https://ansible.fontein.de/collections/ansible/builtin/sh_shell.html#ansible-collections-ansible-builtin-sh-shell (click it!)

[samccann]

Tested locally and looks good except the new index of environement variables isn't part of any TOC that I can see? should be be in the list of 'index of xxx' like modules, vars plugins etc?

We should also add a link to this from here? https://docs.ansible.com/ansible/latest/reference_appendices/config.html#ansible-configuration-settings

[felixfontein]

Tested locally and looks good except the new index of environement variables isn't part of any TOC that I can see?

Yes, it would need to be added there somewhere.

should be be in the list of 'index of xxx' like modules, vars plugins etc?

You mean in https://docs.ansible.com/ansible/devel/collections/all_plugins.html ? It would be a bit confusing if "Indexes of all modules and plugins" also contains an index of all environment variables. (On the other hand, that already contains an index of roles, but roles fit better in there than environment variables :) ) I'm not sure what the best place would be.

We should also add a link to this from here? https://docs.ansible.com/ansible/latest/reference_appendices/config.html#ansible-configuration-settings

That requires a code change in ansible/ansible, but would be a good thing IMO.

[samccann]

docs and output LGTM

[felixfontein]

Ok, plan is to move the 'env variable fallback documentation for modules' to a separate PR so the uncontroversial part can be merged first.

[felixfontein]

I removed the module part (it is now a separate PR: #75).

[felixfontein]

@samccann thanks for reviewing this!