De-duplicate README content and update installation instructions

[matt-graham]

Resolves #264
Resolves #222

• Updates README.md to make it more portable (uses absolute rather than relative links, avoids GitHub Flavour Markdown emoji references in favour of Unicode characters, adjusts some formatting and styling) to allow content to be reused.
• Removes current separate README content for PyPI index page in .pip_readme.rst in favour of just using main README.md and updates docs/index.rst to include README.md content.
• Installation instructions in README.md updates to reflect current approach for installing (optional) dependencies, and old installation instructions in docs/user_guide/install.rst removed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.28%. Comparing base (ea11832) to head (54c7a7d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #274   +/-   ##
=======================================
  Coverage   96.28%   96.28%           
=======================================
  Files          32       32           
  Lines        3606     3606           
=======================================
  Hits         3472     3472           
  Misses        134      134           

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