Improve docs and document contexts

[gdalle]

DI docs

• Move around Markdown files
• Split the content of the "operators" page into "operators", "backends" and "advanced"
• Shave off the details for backend implementations, keep only a summary table of overloads
• Document contexts and Constant with examples
• Remove most backends from docs/Project.toml to allow easy environment resolution

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (267023a) to head (a59b13a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #469   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files         108      108           
  Lines        4297     4297           
=======================================
  Hits         4233     4233           
  Misses         64       64           

Flag	Coverage Δ
DI	98.57% <ø> (ø)
DIT	98.38% <ø> (ø)

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