References #18148.

Link to the "English" section to specify American English be used for the Guides
[ci skip]

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames

Also remove a duplicated sentence.

Added instructions of latest bundler install + modified the on: :save option description in validations

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

In Textile `*` would convert to `<strong>`, but in Markdown we have to
use `**` instead.

[guides] guides:generate:kindle is the new interface to generate .mobi file instead of using KINDLE=1

upgrades RedCloth dependency to ~> 4.2, adds RedCloth to the project's Rakefile, removes gem call from the guides generator (use bundle exec if needed)