Improve docstrings for most commonly used classes and configs

[AdeelH]

Overview

This PR improves the docstrings for the most commonly used classes and configs. Most config docstrings now have a link to their corresponding non-config class.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

While working on this PR, I learned that

:some_domain:`~full.path.to.SomeObject`

can be shortened to

:some_domain:`.SomeObject`

This made adding cross-reference to docstrings a lot cleaner.

Testing Instructions

Check if the build docs look fine.

Part of #1505

[codecov]

Codecov Report

Merging #1630 (e928479) into master (50b7292) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1630      +/-   ##
==========================================
- Coverage   75.71%   75.66%   -0.05%     
==========================================
  Files         192      192              
  Lines        9260     9254       -6     
==========================================
- Hits         7011     7002       -9     
- Misses       2249     2252       +3     

Impacted Files	Coverage Δ
...core/rastervision/core/analyzer/analyzer_config.py	70.00% <ø> (ø)
..._core/rastervision/core/analyzer/stats_analyzer.py	94.11% <ø> (ø)
...astervision/core/analyzer/stats_analyzer_config.py	75.00% <ø> (ø)
...n_core/rastervision/core/backend/backend_config.py	66.66% <ø> (ø)
rastervision_core/rastervision/core/box.py	92.41% <ø> (ø)
...vision_core/rastervision/core/data/class_config.py	92.18% <ø> (ø)
...ision/core/data/crs_transformer/crs_transformer.py	71.26% <ø> (ø)
...sion_core/rastervision/core/data/dataset_config.py	80.70% <ø> (ø)
..._source/chip_classification_label_source_config.py	87.75% <ø> (ø)
...sion/core/data/label_source/label_source_config.py	85.71% <ø> (ø)
... and 64 more

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