Add documentation files in RST format

This is the first step for moving the documentation format from *.txt to the
*.rst format.

In the last years we've been automatically converting txt files from Steve's
markup into RST to generate the documentation pages via Sphinx. The decision
has now been made to fully migrate to RST and avoid the conversion in the
future.

This will enable us to fully use RST and Sphinx to improve the documentation.

For the intermediate period, while there are still pending PRs with
documentation changes, we will keep both source files and update the RST files
accordingly.