Clarify docs links in README

[waldyrious]

• The comparison with curl/httpie is not part of the user guide; moved it to the first paragraph
• Explicitly mention that the user guide describes both installation and usage

[codecov]

Codecov Report

Merging #190 (6e71069) into main (d9c4f96) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #190   +/-   ##
=======================================
  Coverage   74.95%   74.95%           
=======================================
  Files          25       25           
  Lines        3406     3406           
=======================================
  Hits         2553     2553           
  Misses        655      655           
  Partials      198      198           

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