Installation instructions cleanup in User Guide

[namiroues]

As part of the SDG documentation update, duplicate installation instructions from all MDAnalysis documentation sources need to be removed. The goal is to merge all installation instructions into the User Guide.

Steps:

1. Identify all sources with installation instructions.
2. Merge installation instructions into the User Guide.
3. Remove duplicate installation instructions from other sources.
4. Add references linking users to the User Guide.