Documentation: Guide update: Updates to "Basic Scene" Guide

[diarmidmackenzie]

Description:

2nd of what will in fact be 4 updates to bring external glicth examples up-to-date & into the A-Frame repo, per #5207.

• Bring code from Glitch into A-Frame Examples
• Code up-to-date with latest versions
• Code and documentation match

[diarmidmackenzie]

Moved from examples\showcase to examples\docs.

[diarmidmackenzie]

Following up on this PR & also #5211

I believe both can be merged. Unresolved issues that came up in review were:

auto-sync to glitch. Technically complicated. #5222 tracks the options here:
whether to refer to ../../../dist/aframe-master.js or aframe.io/releases/[release]/aframe.min.js - we agreed the best option is to refer to dist, with a comment indicating how to use aframe.io when copy/pasting the code. 5214 and 5215 already merged using this pattern. If we implement sync-to-glitch, it will be possible to adjust the code as part of the sync operation so that it works on glitch without a local aframe library.
#5211 (comment) - unfortunately it doesn't look like any good options available here. As per #5211 (comment), embedme is promising, but will create maintenance pain as code snippets have to be referenced by line number, rather than by tags.
While it would be nice to solve some of the above issues, none is simple to solve, and I don't see a good case for keeping outdated example code in the meantime.

So on the basis that we merged #5124 and #5215 already, I think we should be OK to merge this one & #5211