Document optional extras ([demo], [dev], [docs]) in README install section

[gmarupilla]

The README install section currently shows only:

pip install sarxarray

pyproject.toml defines three useful extras ([dev], [docs], [demo]) but they aren't mentioned anywhere in the README. Users following the tutorial notebook hit ModuleNotFoundError: matplotlib at the visualisation cells because jupyterlab and matplotlib only come in via [demo].

Two small README additions would help:

1. List the available extras and what each one is for.

2. Show the zsh-safe form, since pip install sarxarray[demo] errors out as zsh: no matches found: sarxarray[demo] on the default macOS shell:

   pip install 'sarxarray[demo]'    # to run the example notebook
   pip install 'sarxarray[dev]'     # to run the test suite
   pip install 'sarxarray[docs]'    # to build the docs locally

Happy to send a PR if useful.

(Filed in the context of JOSS review openjournals/joss-reviews#10492.)