docs: reorg Getting Started (English)

This commit makes the HTML and PDF versions of the Getting Started
docs have the same contents, built from the same asciidoc source file.
There are still small wrapper files because the PDF and HTML want slightly
different headers & footers, but the actual contents are unified.

It moves all the Getting Started sub-docs into a single directory.

It removes the per-section html files linked from the main index.html
page and replaces them with a single Getting Started document (which
contains all the sections).

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: reorg Getting Started (French)

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: reorg Getting Started (Spanish)

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>