Clean up docs directory

[scolapasta]

Looking at the docs, we have several things in there that we probably don't need as "official" docs for release.

Let's review and clean this up.

Example:
the /doc/gui/ingest/language_encoding/ directory.

[scolapasta]

Handing first to @landreev, but this may need to be reviewed by several developers.

[landreev]

I cleaned the doc files that were mine, that were no longer needed.