Clean up the repo to facilitate distribution via Conda-Forge

[gferragu]

After discussion with Eric and Mike, removing the CLI program seemed like the easiest way to allow for gmpacket to evade the seis-prov dependency that's causing the Conda recipe errors when checking the entry points.

The validate and scan modules remain, however, and can still be used in a Python script provided that seis-prov is installed locally.

Changes:

• Remove gmpacket/gmpformat.py and gmpacket/bin/gmpformat,py that calls it
• Update pyproject.toml to reflect the removed CLI entry points
• Update documentation to remove references to the CLI usage, redirect links to the demo towards the newer "Pydantic Demo"
• Streamline dependencies in pyproject.toml by moving document-related packages into [doc] and build-related packages into [build]
• Update documentation to reflect the optional dependency installations, which are now described in a page called "Getting Started"

[gferragu]

@emthompson-usgs , @mhearne-usgs, would you mind looking over this and seeing if things are to your satisfaction?