chore: run ci on push to main, exclude notebook tests on push

[tomjholland]

This pull request includes several changes to the CI workflow configuration files, primarily focusing on improving the build and test process and removing redundant steps.

Changes to .github/workflows/ci.yml:

• The workflow is now triggered on pushes to the main branch and on pull requests targeting the main branch. ([.github/workflows/ci.ymlL4-R6](https://github.com/ImperialCollegeLondon/PyProBE/pull/205/files#diff-b803fcb7f17ed9235f1e5cb1fcd2f5d3b2838429d4368ae4c57ce4436577f03fL4-R6))
• Removed the step for uploading the coverage artifact, as it is no longer needed. ([.github/workflows/ci.ymlL39-L44](https://github.com/ImperialCollegeLondon/PyProBE/pull/205/files#diff-b803fcb7f17ed9235f1e5cb1fcd2f5d3b2838429d4368ae4c57ce4436577f03fL39-L44))
• Added a condition to run notebook tests only on pull requests. ([.github/workflows/ci.ymlL59-R56](https://github.com/ImperialCollegeLondon/PyProBE/pull/205/files#diff-b803fcb7f17ed9235f1e5cb1fcd2f5d3b2838429d4368ae4c57ce4436577f03fL59-R56))

Changes to .github/workflows/coverage.yml:

• Removed the entire workflow for uploading coverage on PR merge, as it has been integrated into the main CI workflow. ([.github/workflows/coverage.ymlL1-L25](https://github.com/ImperialCollegeLondon/PyProBE/pull/205/files#diff-a2115d277b5ca5a2f09a999e53440839cf332b94da177f3d1766334555b0f7c6L1-L25))

[codecov-commenter]

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Additional details and impacted files

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