Add InterSphinx Support and Switch from autoapi to autodoc

[medley56]

Intersphinx allows linking between documentation sources generated with Sphinx and autodoc. The documentation generated for space-packet-parser should include the intersphinx extension so that other documentation sources can link to it.

As part of this work, the documentation generator for space-packet-parser should also be switched to the built in autodoc/autosummary instead of the separate sphinx-autoapi package.