Fix the usability of templateFolder and outputFolder

Description

This updates the templateFolder and outputFolder helpers to return absolute paths instead of the raw, relative paths. Using relative paths breaks the intended usage of these functions, which is to create relative paths in templates. Specifically, these functions return garbage values when using relative path inputs to --template-url and --output-folder, as these get repeated in the boilerplate blueprints, leading to broken relative pathing.

Using absolute paths ensure that the relative paths constructed in the boilerplate templates always point to the intended locations.

TODOs

Please ensure all of these TODOs are completed before asking for a review.

[x] Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
[x] Update the docs.
[x] Keep the changes backward compatible where possible.
[x] Run the pre-commit checks successfully.
[x] Run the relevant tests successfully.

Note on backward compatibility

I intend to release this as backward incompatible release given that the output of templateFolder and outputFolder changes, which will break anything that renders it in, say, docs, but for most use cases of those helpers, this should be functionally equivalent. E.g., I don't expect these changes to break our Reference Architecture.

All this is to say that there will be an explanation, but no migration guide will be provided given the functional equivalence.

gruntwork-io / boilerplate