Inventory: Implement auto-discovery of conf.py

[amotl]

About

From the Linksmith RFC:

• sphobjinv doesn't know how to introspect conf.py:intersphinx_mapping,
  to go from project name to docset URL.

/cc @bskinn, @chrisjsewell

[codecov]

Codecov Report

Attention: Patch coverage is 97.91667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.57%. Comparing base (c520b65) to head (92a7cc9).

Files	Patch %	Lines
linksmith/sphinx/core.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   99.77%   99.57%   -0.20%     
==========================================
  Files          11       11              
  Lines         440      475      +35     
==========================================
+ Hits          439      473      +34     
- Misses          1        2       +1     

Flag	Coverage Δ
unittests	99.57% <97.91%> (-0.20%)	⬇️

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[amotl]

• linksmith 0.0.1 has been released.
• Minimal usage docs at https://linksmith.readthedocs.io/en/latest/usage.html.
• Please create issues for any other wishes in this regard, both on ingress flexibility, and output formatting. Thanks!