Streamline README and move long usage notes into docs

[Erotemic]

The current README is too long. This is a move to a shorter README while keeping the old one for now. The idea is that things from the legacy README should move into a proper place in the docs over time, but for now nothing is lost on the public facing code, and it pushes a cleaner first impression now, and lets us decide what that logical docs split is later.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.95%. Comparing base (b5ca752) to head (307f4b9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #435   +/-   ##
=======================================
  Coverage   84.95%   84.95%           
=======================================
  Files          21       21           
  Lines        2412     2412           
  Branches      376      376           
=======================================
  Hits         2049     2049           
  Misses        263      263           
  Partials      100      100           

Files with missing lines	Coverage Δ
...ne_profiler/autoprofile/ast_profile_transformer.py	84.37% <ø> (ø)
line_profiler/autoprofile/line_profiler_utils.py	52.94% <ø> (ø)
line_profiler/cli_utils.py	89.89% <ø> (ø)
line_profiler/ipython_extension.py	98.19% <ø> (ø)
line_profiler/line_profiler.py	95.77% <ø> (ø)

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