docs: split apart installation docs into parts

[jackgerrits]

Why are these changes needed?

Currently the installation page has a lot of information that is valuable but hard to read through. This splits apart that info and organizes it into several pages (including nesting) to make it easier for the user to find what they are after

Split installation into:

• Installation (venv and install)
  • Docker
  • Optional deps
• Migration guide

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd0094c) 32.48% compared to head (88c73d0) 32.48%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1420   +/-   ##
=======================================
  Coverage   32.48%   32.48%           
=======================================
  Files          41       41           
  Lines        4907     4907           
  Branches     1120     1120           
=======================================
  Hits         1594     1594           
  Misses       3187     3187           
  Partials      126      126           

Flag	Coverage Δ
unittests	32.44% <ø> (ø)

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[afourney]

Code formatting fails on an unrelated notebook file:

nbqa-black...............................................................Failed

Which I cannot reproduce locally to correct. See #1418 for discussion.