- Filling out index descriptions for `using_yosys` and `using_yosys/synthesis`.
- To discourage skipping over these index pages, the toctree in
`using_yosys/index` is hidden and instead has inline links to the two
subsections.
- Tidying todos.
- Moves technology mapping to `techmap_synth`, leaving the techmap by example in
the internals section. `yosys_flows` gets split up, with the coarse-grain
intro replaced by `synthesis/index`, the extract pass moving to
`synthesis/extract` and model checking to `more_scripting/model_checking`.
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