Generate README.md for a catalog collection

[hombit]

Initial implementation of #615 (not closing it yet).

Description

Adds hats.io.summary_file.write_collection_summary_file method to create a catalog root "index" file with a catalog collection description. Currently it supports fmt="markdown" only, but we also plan to add "html" (for HTTP storage, like data.lsdb.io/hats//index.html) and "json" (machine-readable, e.g., for data.lsdb.io generation) formats in the future.

Currently, the generated README.md is very minimalistic and includes three parts only: optional Hugging Face metadata (to enable UniverseTBD/mmu-hdf-to-hats#30), title, and a very short description. This is to be extended in future PRs.

New dependencies

Both are for the Hugging Face functionality

• jinja2 is added as a "main" dependency, which adds two packages to our dependency tree
• pyyaml is added as a [dev] dependency, but it is actually already included with astropy
• datasets is added to requirements.txt for testing Hugging Face metadata. I tried to add it to [dev], but I cannot make it work with test-lowest-deps. Let me know if there is a better solution

[github-actions]

Before [ac210ed] <v0.8.0>	After [64c9622]	Ratio	Benchmark (Parameter)
42.2±0.4ms	43.4±0.7ms	1.03	benchmarks.Suite.time_pixel_tree_creation
12.4±0.3ms	12.5±0.3ms	1.01	benchmarks.Suite.time_inner_pixel_alignment
279±2ms	281±3ms	1.01	benchmarks.time_open_large_catalog
403±1ms	401±2ms	1	benchmarks.Suite.time_outer_pixel_alignment
278±2ms	277±1ms	1	benchmarks.time_small_cone_large_catalog
121±1ms	121±0.6ms	1	benchmarks.time_test_alignment_even_sky
1.04±0.03ms	1.03±0.02ms	1	benchmarks.time_test_cone_filter_multiple_order
45.2±0.2ms	44.6±0.1ms	0.99	benchmarks.time_open_midsize_catalog

Click here to view all benchmarks.

[codecov]

Codecov Report

❌ Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.50%. Comparing base (ac210ed) to head (c43e96a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hats/io/summary_file.py	93.18%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   93.51%   93.50%   -0.01%     
==========================================
  Files          50       51       +1     
  Lines        2450     2494      +44     
==========================================
+ Hits         2291     2332      +41     
- Misses        159      162       +3     

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