Introduce CI workflow logic

[jwallwork23]

Closes #194.

In the interests of green computing and avoiding hitting the limit of the free-plan, this PR introduces some logic to only run workflows if relevant changes are made.

• Only run the tests if source files, CMakeLists.txt, requirements.txt, or data are changed.
• Only run the static analysis workflow if C, C++, Fortran, or Python files have changed.
• Only run the Fypp checks if Fortran files have changed.
• Only build the docs if Markdown or content in pages/ have changed.

If any of the workflow files are changed then those tests should also be run.

The PR also improves the consistency of formatting across the workflow configuration files.

[jwallwork23]

Some checks:

• My initial push 00bee3e didn't run any tests because no source code or docs have been changed.
• My second push de42e78 added a check for if fypp_checks.yml changed. This file has changed in the PR so those tests were run.
• My third push d154d14 added a check for if static_analysis.yml changed, which it has. So both Fypp and linting checks were run.
• (You get the picture) 3ba39ca added build_docs.yml check, so Fypp, static analysis and build docs were run.
• Finally c1f6a04 added test_suite.yml check, so all four were run.

[jatkinson1000]

This all seems sensible @jwallwork23
A couple of stylistic comments and a suggestion to remove the options you had already flagged.

[jatkinson1000]

LGTM, have added suggestions for formatting of the other branch coverage lists.