Expand margin generation explanation. Use anon references.

[delucchi-cmu]

Expands the description of margin generation:

• link to LSDB margin demo notebook. gives nice motivation.
• show the order to mindist lookup table
• give visual on margin pixels
• remove text about precise margin filtering, as it has been disabled.

Switches external links to use the dunder anonymous reference style. See also astronomy-commons/hats#454

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.97%. Comparing base (201397c) to head (dc90bd1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #485   +/-   ##
=======================================
  Coverage   94.97%   94.97%           
=======================================
  Files          28       28           
  Lines        1811     1811           
=======================================
  Hits         1720     1720           
  Misses         91       91           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.