Abandoned documentation removal

[zregvart]

With the documentation generation in #1500 and #1493 the .adoc files are generated in docs/modules/ROOT/pages. A document created prior to it's abandonment, for example if a command or a Rego builtin is removed, will not be removed.
The document generation code should keep track of any abandoned components and remove the corresponding Asciidoc files.

[simonbaird]

The lazy way would be to rm -rf all the generated files before regenerating them.

[zregvart]

The lazy way would be to rm -rf all the generated files before regenerating them.

Yes, but we need a list of the ones we can remove, so fist step would be to figure out what .adoc files are generated vs which ones are handwritten. They're all in the same directory currently, so perhaps separating them somehow would be a good start.

The nav.adoc files get regenerated so we don't need to remove those.