Devise mechanism for importing ReST documentation

[nottrobin]

As previously discussed, both MAAS API docs & Landscape API docs are currently created as ReST files. The simplest way to then include these in this site is to parse those files into Markdown and add them to the existing documentation sets.

It would be nice if this could be an entirely scripted task so it's easy to update them as often as we need. A starting point is (from @degville's work):

pandoc --from=rst --to=markdown+backtick_code_blocks+pipe_tables+definition_lists+compact_definition_lists --column=78 --atx-headers --output output.md input.rst

The above command still has the following problems:

• lists are preceded with a redundant '>' because they're indented in
  the original rst document.
• image path needs to be './media'
• most internal references don't work

I also have a worry about doing this, as also articulated on canonical/ubuntu-core-docs#8, that this breaks the purity of the relationship between the built documentation and the markdown repository. This still needs to be discussed further.

[nottrobin]

This issue was moved to canonical/documentation-builder#90