Documentation: Recommend the official docker image in docs and improve structure of installation intro

[mkbrechtel]

The docker.io/monarchinitiative/linkml OCI image has last been updated 3 years ago and there is an official docker image. This should be mentioned in the docs.

Also i took the opportunity to improve the structure of the installation guide a bit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.04%. Comparing base (2ede961) to head (3e84c2d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2073   +/-   ##
=======================================
  Coverage   80.04%   80.04%           
=======================================
  Files         109      109           
  Lines       12168    12168           
  Branches     3449     3449           
=======================================
  Hits         9740     9740           
  Misses       1852     1852           
  Partials      576      576           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.