Unify docs build into a single build, support proper multiversion links

[adelbertc]

Description

• Unify Sphinx build to build as one project instead of separate User Guide and Deployment Guide builds
• Change hard-coded URLs to use Sphinx roles so the correct multiversion link can be interpolated at build time
• Add some overrides to nvidia-sphinx-theme hardcodes to get the correct sidebar Table of Contents behavior after unifying the build and by extension the toctree

The goal of this change is to make the links in our docs version-aware after moving to multiversion docs in #40. Normally the way to do this is to reference roles like :doc: and :ref: instead of hard-coding URLs, but since we have cross-guide links we also have to unify the Sphinx build to make the builds aware of roles across all our docs.

NOTE: Our root README and similar docs still hard-code to main. I figure this is an acceptable tradeoff at not complicating the build to account for this. All links within the actual docs pages should be cromulent.

Issue #92

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.