Embed and build the inkscape manual

Summary:

It has been suggested to me in https://bugs.debian.org.org/1004062 - that indeed it would be best to improve the Help → Manual experience.

I could probably cook up something on the packaging side, but I believe it would be best if inkscape could ship with that manual built-in instead (perhaps still with a button to access the online version. But changing the current one, that is just way too outdated).

Let me just copy over the report:

Currently Inkscape's Help menu just opens URLs.

https://sources.debian.org/src/inkscape/1.1.1-2%7Ebpo11+1/src/verbs.cpp/#L2051-L2101

My disadvantaged users do not have internet access, so currently they cannot get help. This is already fixed for Help > Tutorials (apt install inkscape-tutorials). Can this be extended to the rest of the Help menu?

The tutorials come from this repository:

https://gitlab.com/inkscape/inkscape-docs/documentation

Help > Keys comes from that same repository. Help > Command Line Options comes from that same repository (./man/).

Help > Manual currently points to an third-party manual that is

• written for the wrong Inkscape version,
• doesn't support https, and
• cannot be distributed by Debian (non-commercial clause).

However, there is a new first-party manual which is here:

https://gitlab.com/inkscape/inkscape-docs/manuals

It is available pre-rendered here (only for latest version, not guaranteed to be for the version Debian ships):

https://inkscape-manuals.readthedocs.io/en/latest/
https://inkscape-manuals.readthedocs.io/_/downloads/en/latest/pdf/
https://inkscape-manuals.readthedocs.io/_/downloads/en/latest/htmlzip/
https://inkscape-manuals.readthedocs.io/_/downloads/en/latest/epub/