Improve docs

* README.md
* [ ] Add introduction from docs/introduction.rst to README.md as currently intro is a bit short.
* [ ] Move client site to top of usage as most users will use client
* [ ] Link TLDR code snippets to doc website for more info.
* [ ] Add code of conduct
* [ ] Add contributing docs
* [ ] Move README.md#development-generating-the-grpc-code
* [ ] How to run tests #80
* [ ] How to make new release + publish to pypi
* docs/
* [ ] Use sections of README on index page, for example see https://github.com/phenology/springtime/blob/main/docs/index.md
* [ ] Add links from rst to apidocs for even more examples or move examples to rst files.
* docstrings
* [ ] create_grpc_channel + get_unique_port in BmiClient
* [ ] get_value_ref in BmiClientApptainer

Start by reviewing README.md, docs/introduction.rst, and docs/index.md, then inspect the listed BmiClient and BmiClientApptainer docstrings. Break the checklist into a focused scope and verify the relevant documentation, testing, release, and docstring updates are complete.

Written by the indexing model from the issue text.