[Docs Tracking] Docs todo

[Buckminsterfullerene02]

Will update as I think of things that need to be done.

• Settle officially on a versioning system, like semver
• Once we have an official versioning system, explain ABI, and which version number changes are expected to break it and thus require mods to be rebuilt, and that mod creators should develop mods based on the latest non-breaking version.
• Explain difference of xmake f -m and xmake/xmake build. Some users have been confused that the former command "doesn't work" because they aren't aware it is only for setting configuration
• Fill out contributing page (and maybe include in repo root as well). This should include; how to make changelog entries, how PR and release system works, using clang format, how to update docs, etc.
• Update build instructions with -y on commands because minor QoL
• Explain how C++ devs can use xmake.lua to; setup cmake projects, include headers libs, include SDKs, link dlls, etc. Provide examples. If we expect developers to learn a new build system we should make it as easy as possible for them by providing our own docs rather than relying on xmake docs and having to figure out how to apply it to ue4ss

[bitonality]

Adding these commands that are now available as a part of #494 but aren't being advertised publicly yet.

Install a mod to a game.
xmake install -o "C:/MyGame/" <mod_name>
Install all mods to a game.
xmake install -o "C:/MyGame/" -g mods
Currently installing mods also automatically installs the locally compiled version of UE4SS. I'm looking into a way to explicitly disable this option if you want to test mods against a released version of UE4SS...

I also added the ability to install arbitrary files alongside the mod by using

target("mymod")
    add_extrafiles("someconfig.json")

The xmake install command will automatically pick these files up and install them with the mod dll.

xmake install -o "C:/MyGame/" UE4SS
Installs dwmapi.dll, UE4SS.dll and UE4SS.pdb to the game folder.

It's on my radar to remove the now duplicated functionality from UE4SS-RE/UE4SSCPPTemplate#2 since the mod install + ue4ss install logic is now baked in to UE4SS.

[UE4SS]

Updated the first checklist entry, and added another related one.
I think this makes more sense, but feel free to revert or make whatever changes.

[Buckminsterfullerene02]

Added part about linking and such. Any other examples we should provide?

[bitonality]

Explain difference of xmake f -m and xmake/xmake build. Some users have been confused that the former command "doesn't work" because they aren't aware it is only for setting configuration

https://github.com/UE4SS-RE/RE-UE4SS/pull/516/files?short_path=b335630#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5

This PR adds some richer xmake documentation. We should probably decide what should live in the readme and what should live in the template repo/UE4SS docs dir. I figure I'd start with the documentation in the readme since it flows well with the current dev environment set-up steps.