WIP: Adding Getting Started section to docs

[dotsdl]

This PR adds a Getting Started section to the alchemlyb docs. It moves the installation instructions under this heading, and creates new Tutorials and Examples pages under this heading as well.

To give the existing package documentation a clear entry point to relay high-level package details, this PR also adds an Overview of the alchemlyb library page. This is referenced by the Tutorials page.

The following are required to close this PR.

• Completed Pandas Basics tutorial
• Completed 10 Minutes to alchemlyb tutorial
• Completed Overview of the alchemlyb library package documentation page

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (7fcdf7a) to head (8b4e44f).
⚠️ Report is 229 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #99   +/-   ##
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  Coverage   97.26%   97.26%           
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  Files          12       12           
  Lines         695      695           
  Branches      141      141           
=======================================
  Hits          676      676           
  Misses          5        5           
  Partials       14       14           

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