Set up new trusted publication workflow

[davidorme]

Description

This PR:

• Set up a new GitHub Actions workflow publish.yml to publish virtual_ecosystem to PyPI when new GitHub release is published.
• This uses PyPI trusted publishing, so PyPI auth tokens need to be set up or shared.
• Updates the existing test workflow in ci.yml so that it can be re-used as the first step of the the publish.yml workflow.
• Adds a documentation page to the website describing the whole release process.

Fixes #405 (issue)

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist

• [ ] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [ ] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.11%. Comparing base (a204557) to head (5db65c0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #408 +/- ## ======================================== Coverage 94.11% 94.11% ======================================== Files 68 68 Lines 3434 3434 ======================================== Hits 3232 3232 Misses 202 202 ```

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