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 82.40%. Comparing base (c2a6a30) to head (5d0ee19).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
+ Coverage   82.38%   82.40%   +0.01%     
==========================================
  Files          20       20              
  Lines        2254     2256       +2     
  Branches      359      359              
==========================================
+ Hits         1857     1859       +2     
  Misses        300      300              
  Partials       97       97              

Files with missing lines	Coverage Δ
...ne_profiler/autoprofile/ast_profile_transformer.py	47.72% <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)

... and 2 files with indirect coverage changes

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