Help menu: link to online manual only, remove obsolete 2.2 manual

[ronso0]

No description provided.

[Be-ing]

We should update the manual build instead of removing it.

[ronso0]

IIRC we agreed on not delivering the 3.6MB manual with the binaries?
(the manual is not mentioned anywhere in /packaging/ therefore I seem to recall correctly)
Btw, currently even opening the local manual is broken on Linux.

With the web version we can keep updating and improving the manual without users requiring to update, and we probably won't make a point release just for an updated manual.

[Be-ing]

IIRC we agreed on not delivering the 3.6MB manual with the binaries?

Huh? Where was that discussed?

Viewing the manual should not require Internet access. The current situation requiring manual rebuilds of the PDF is hacky but we won't be improving that until 2.4 with #3047.

[ronso0]

I thought that was discussed when @hozhaus revamped the entire manual process.

[Holzhaus]

We could include the PDF manual by adding a CMake option for it that fetches the PDF from downloads.mixxx.org at configure time. But we can also not include it, I'm fine with it either way.

[ronso0]

okay, nevemind.

https://github.com/mixxxdj/mixxx/blob/main/src/widget/wmainmenubar.cpp#L574
if (resourceDir.cd("../doc/mixxx") && resourceDir.exists(MIXXX_MANUAL_FILENAME)) {

this fails because the manual is compressed and installed as Mixxx-Manual.pdf.gz

continued in https://mixxx.zulipchat.com/#narrow/stream/109176-documentation/topic/2.2E3.20manual.20PDF