Document templates better

Today we document that it's possible to provide templates instead of the auto-generated ones, but we don't provide:

• A documentation showing how those templates look like
• Don't mention explicitly that, if used, then it's entirely up to the user what goes in there

@detro I use the generate arg --website-temp-dir to see the default generated templates.

Example: tfplugindocs generate --website-temp-dir /tmp/blah101

You'll find the *.md.tmpl files in /tmp/blah101. I then can decide what I want to tweak in my /templates/[resource/data-source/*.md.tmpl files.