document docs install & dev process

docs/README.md

+Install `mkdocs` and `mkdocs-material` with `pip`.
+
+```
+pip install mkdocs mkdocs-material
+```
+
+To build the documentation, run `mkdocs build` in the root directory of the repository.
+
+To serve the documentation locally, run `mkdocs serve` in the root directory of the repository.
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