Added GitHub workflow to check for broken links

[dc2917]

Description

This PR adds a new GitHub workflow to check the links in the MUSE readthedocs.io documentation. This is helpful since URLs often stop working as time goes by - this therefore provides an automated way of monitoring for those and flagging them to be sorted.

Fixes #102

Type of change

Please add a line in the relevant section of
CHANGELOG.md to
document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python setup.py build_sphinx

Further checks

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