[Docs] Make page `variables.html` into its own directory with more detailed and accurate sub-pages

[Luna-Klatzer]

Is there an existing issue for this?

• [X] I have searched the existing issues

Current State & Issue

The current page variables is rather old and outdated, as it was added in the first dev release of the Kipper docs. Until now only minor changes were performed to keep it accurate, which means most of the layout is still as it was originally. This can lead to confusion, as for example the constant page is at the bottom of the variables page without any indication of it being there.

As such it would be best to split up the page into multiple different pages, which should also cover more edge-cases and be more detailed, as the current variables page is rather generic and not very clear about some behaviour.

Expected Changes & Improvements

• [ ] Split up the variables docs page into multiple new pages.
• [ ] Add more details to every page to make them more clear.
• [ ] Add more examples of variable usage.

Referenced sites

• https://kipper-lang.org/docs/variables.html
• https://kipper-lang.org/docs/latest/variables.html
• https://kipper-lang.org/docs/next/variables.html