Add offline doc how-to in README

[dheavy]

Describe the changes you have made:

Adds to the README a section on how to run the documentation offline.

Reference any relevant issues (e.g. "Fixes #000"):

Closes #1112

Pre-Submission Checklist (optional but appreciated):

• [x] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [x] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [x] Tested on Windows
• [x] Tested on MacOS
• [ ] Tested on Linux

[MikeBirdTech]

Thanks @dheavy! This looks great. I'll let @KillianLucas make the final decision but 👍 from me

[KillianLucas]

Great work @dheavy! Love to imagine folks cloning the repo, getting into OI away from an internet connection with nothing but this and a local LLM. Merged.

[dheavy]

@KillianLucas that's literally me a couple of weekends per quarter in the mountains!