MkDocs generator only deploys on tag

When running the action with `generator: sphinx` (default value), documentation is generated and deployed to the `gh-pages` branch in two cases:

A. When the commit that triggered the build is on the `main` branch.
B. When a label is created (for example, `v1.2.3`).

This logic is guarded by this check:
https://github.com/compas-dev/compas-actions.docs/blob/250701684f9eefb501c8e46933282da284cdf02a/action.yml#L136

However, when running the action with `generator: mkdocs`, the generated documentation is only pushed when a label is created, **ignoring commits pushed to `main`**. This check occurs in line 161:
https://github.com/compas-dev/compas-actions.docs/blob/250701684f9eefb501c8e46933282da284cdf02a/action.yml#L153-L167

## Expected behavior

The action should deploy docs under the same conditions, regardless of which generator is used. Commits pushed to `main` should trigger a deploy to `gh-pages` when using the MkDocs generator.