Add a getting started page

[jeremykubica]

Add a very basic getting started page to HATS.

[github-actions]

Before [ffae445]	After [237e9a8]	Ratio	Benchmark (Parameter)
13.0±0.2ms	14.0±0.8ms	1.08	benchmarks.Suite.time_inner_pixel_alignment
45.0±1ms	46.9±0.8ms	1.04	benchmarks.Suite.time_pixel_tree_creation
16.8±0.1ms	17.1±0.2ms	1.02	benchmarks.MetadataSuite.time_load_partition_info_order6
89.0±2ms	90.6±3ms	1.02	benchmarks.Suite.time_paths_creation
958±7μs	978±20μs	1.02	benchmarks.time_test_cone_filter_multiple_order
72.2±0.6ms	71.9±0.5ms	1	benchmarks.MetadataSuite.time_load_partition_info_order7
71.8±0.5ms	72.0±0.5ms	1	benchmarks.MetadataSuite.time_load_partition_join_info
348±1ms	348±3ms	1	benchmarks.Suite.time_outer_pixel_alignment
126±0.5ms	126±2ms	1	benchmarks.time_test_alignment_even_sky

Click here to view all benchmarks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.75%. Comparing base (ffae445) to head (9fe3baf).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #382   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files          48       48           
  Lines        1933     1933           
=======================================
  Hits         1793     1793           
  Misses        140      140           

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