These references haven't been visible for years.
Related to:
- https://github.com/gohugoio/gohugoioTheme/issues/24
- https://github.com/gohugoio/gohugoioTheme/pull/216
While Hugo does not care if the section is named "post" or "posts",
this commit normalizes the whole Hugo documentation to use "posts"
when used as an example for a section name to prevent user confusion.
Fixes https://github.com/gohugoio/hugoDocs/issues/720.
Also highlight the use of `site.Params.mainSections`.
