Improvements in docs generating

[t0mk]

This PR contains improvements in doc generating, it

• updates ansible-specdoc to 0.0.13
• adds inject-module-docs and remove-module-docs targets to Makefile. The targets will fill DOCUMENTATION, EXAMPLES and RETURN from ansible-specdoc configurations
• switches doc template for inventory plugin to RST instead of markdown
• replaces mentions of metal_auth_token with metal_api_token
• removes ansible-specdoc usage from metal_device inventory. This creates problems because inventory functionality is affected by it's DOCUMENTAION.
• injects generated docs to some of the modules
• fixes typos

This should have been split in more PRs, sorry about it, I'll pay attention better next time.

[t0mk]

@ctreatma please take a look when you have time. We can try to do a release once this is in.

[t0mk]

@ctreatma thanks for approving. I will fix the tests and merge.