Make manual downloads more visible

[leamas]

Is your feature request related to a problem? Please describe.
The User Manual is available as a download bundle, but this is not that visible. As for the Plugins Manual there is no download bundle available. Hence, in off-line scenarios which is a design goal manuals tends to be unavailable.

Describe the solution you'd like
Make both the User Manual (UM) and the Plugins Manual (PM) available for download using already existing mechanisms like plugin manager or chart downloader.

Describe alternatives you've considered
See above

Additional context
A UM bundle already exists. A PM bundle does not exist, the first step would be to create this. This could be either in as HTML or pdf, both have their pros and cons.

Making this as plugins is sort of attractive: the manuals would then be visible in the catalog, updates would be handled, etc. There is an all ABI type yet not used which could make it possible to create a single plugin to be used on all platforms. It is however unclear how a plugin without an actual library would fare in current structure.

The UM bundle is not distributable in Linux packages since it is not open source.