Update docs

[ffreyer]

Let's try this again. I'll won't try to be thorough with this, otherwise it'll just sit around forever and go stale again.

• [x] move old docs onto a different page
• [x] add Introductory DQMC page
• [x] add lattice docs
• [x] add docs for dqmc models
• [x] add docs for DQMC
• [x] add docs for configuration recorders & compression
• [x] add scheduler docs
• [x] add measurement docs
• [ ] maybe also some FileIO information