Overhaul "Install" instructions

[avivace]

I wasn't quick enough to review #47, so let me try again.

The install page suffers from a lot of problems right now, making the whole thing unreadable and inaccessible to the average target of "new users of RGBDS".

Let's try to review the criteria:

• [x] Keep the details to the minimum. We are not interesting on explaining the intricacies of installing on Windows or in which way different ways of installing on Windows are related.
• [x] Our primary dev environment (and the largest RGBDS user quota) target is "Linux". We have pre-built binaries for amd64 on the CI (?). Until we start packaging it for main distributions, that should be the primary suggested way to obtain the binaries. End of the story.
• [x] Reading the install page is incredibly complex right now and leads to so many choices that it makes me want to give up just looking at all the branches and choices I have to make.

Feel free to discuss any of these points. In the meantime, I started an overhaul:

• Removed mention of rgbds2 package. Why would someone be interested in it? It will just make the explanation and the options more complex for new users. If I need an old version, I'll look up for an old release, no need to imply 3 choices in the Arch Linux tab. it's not an alternative/LTS branch receiving updates, so it's not like I need to subscribe to a rgbds2 package.
• Made a generic "Linux" tab as the default. Go to releases, pick a pre-built binary for the arch you need. You're done.

Fixes #51.

[ISSOtm]

Are you sure the largest portion of our user base is on Linux? Lacking any better info (people will get mad if we add any telemetry), I've been referencing the Sublime Text extension's stats, which are approximately 50% Windows, 25% macOS, 25% Linux. [EDIT] Since Sublime is non-free software, and not in any distro's default repos, maybe the Linux share is biased downwards. IDRK.

As far as I'm aware, distributing raw Linux binaries is a bad idea because they'll be incompatible across distributions? I will look up info on that.

[avivace]

Let's also remember to mention rgbenv

[avivace]

This is currently blocked by https://github.com/gbdev/rgbds/pull/1148 not being applied to old releases, so we don't have the Linux build until we release

[avivace]

https://github.com/gbdev/rgbds-www/issues/51 could also be fixed here

[ISSOtm]

I'm not sure Docker would be much use for installing RGBDS?

[pinobatch]

It has been suggested to use https://nightly.link/ to solve the lack of guest downloads for master.

[avivace]

@ISSOtm

I'm not sure Docker would be much use for installing RGBDS?

The container image provides a very quick way, literally a oneliner (if you have a container runtime installer ofc) to obtain a linux environment able to run AND build RGBDS (the version you're checking out, but potentially any other).

It may also be useful as a "base" image for pipelines.

Anyway, we'll have some stats in a while on how popular/really used really is, so I'm okay in removing it from the options if it we understand is not really used..

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Another thing, are we really sure we want the Tabs navigation here? Now the instructions were slimmed down, they seem to amount just 4-5 lines per platform so I'm now sure if requiring additional clicks is really useful?

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@PinoBatch where should the nightly link https://nightly.link/gbdev/rgbds/workflows/testing/master go? In the "Install master?" (https://rgbds.gbdev.io/install/master) (which could also be renamed to "Canary" tbf, it's clearer