Documentation: solver method guides for docs site

[jc-macdonald]

Create documentation-site guides for solver methods rather than embedding long descriptions in README.md or shipping raw example scripts.

Scope

• Write solver method usage guides for https://accidda.github.io/op_engine/latest/
• Cover the biogeochemical network example (examples/biogeochemical_network.py) as a tutorial-style guide
• Cover the split diagnostic example (examples/biogeo_split_diagnostic.py) similarly
• Keep README.md descriptions brief with links to docs

Context

Tim suggested the README additions in PR #18 would be better as standalone docs-site guides. The examples directory files should be referenced from documentation rather than being standalone scripts.

Ref: PR #18 review by @TimothyWillard (README.md:33)