Create .documentationrc file to sort project settings

When building documentation each project requires a number of flags to be set. For example:
```bash
documentation-builder --base-directory ../vanilla-brochure-theme/docs \
--site-root '/en/vanilla-brochure-theme/index' \
--output-path 'en/vanilla-brochure-theme' \
--template-path template.html \
--tag-manager-code 'GTM-K92JCQ' \
--no-link-extensions \
--force
```
This is difficult to type/remember so the command is normally added to a scirpt like `./build-docs`. This makes it difficult to modify the flags like changing the source-directory in a one off case.

I propose that the settings can be stored in a `.documentationrc` file which is committed to the project and is looked up when running `documentation-builder`. Therefore you only need to run `documentation-builder` in the project with overriding flags.

Start at the documentation-builder command entry point and compare its flags with the example ./build-docs script. Check how a committed .documentationrc would be discovered and how command-line overrides should behave. Done means the project can run documentation-builder with stored settings while allowing individual flags to override them.

Written by the indexing model from the issue text.