Removes htmlify.p6

Its functionality has been superseded by the new Documentable. It's had a nice run, but previously I deleted a couple of libraries that depended on it, so it was no longer functional.

Refs #2996. But also:

* Closes #2883 since disambiguation files are no longer generaed.
* Closes #2801 since that's been fixed in documentable.
* Closes #2592 since it works correctly in documentable.
* Closes #2575 since indexing is now specified by a grammar in documentable.
* Refs #2542: this is an ongoing work.
* Closes #2539: initial tests were mede in a mini-docs site, then a staging site, then in production.
* Closes #2529: mini-docs were used in the process of the new documentable, https://github.com/antoniogamiz/mini-doc/
* Closes #2499: Disambiguation is deprecated #2974 and is currently eliminated from production.
* Closes #2498: It takes now around 15 minutes (in staging site) to generate documentation the first time, less than a minute to do it; 8 minutes in CircleCI.
* Closes #2392: Now pages include metadata, and it's been resigned and disambituated.
* Closes #2270: fixed in documentable.
* Closes #1973, documentable now does not generate temporary files.
* Closes #1952: caches can now be shared, since there exists one. However, it's not shared yet between pod2onebigpage, for instance.
* Closes #1937, since this has been already spun off and released to the ecosystem
* Closes #1854: fixed either here or in Documentable
* Closes #1823: It's totally refactored and working independently.
* Refs #516: the new Documentable allows, by specification, several types in the same page.