Re-organize Getting Started and Docs

[vresbok]

Re-organization after discussion last TreePPL meeting:

• Move all pages under Language overview one level up.
• Move all pages from Getting started to Documentation.
• Link Getting started in top bar directly to Getting started under Documentation.
• Delete Python environment page.

ToDo still:

• @ThimotheeV can send info on Distributions missing from the list.
• Question: Is this warning still accurate:
  `We currently support Linux and MacOS only. For Windows users there are two options: use a virtual machine/server/cluster or use TreePPL with WSL (see [Windows installation instructions])

[elegios]

The warning is indeed still accurate, though we might want to phrase it to connect more to the treeppl-python/treepplr wrappers. For example, python needs to be run under WSL and not natively to use treeppl-python on windows.

[kudlicka]

In the description above, the link includes http://localhost:3000, which points to a local development server. The link should be /docs/for-developers/install_windows only.

[vresbok]

The http://localhost:3000 only shows because I copied this while building locally - sorry about the confusion. There is no such link local on the live webpage.