Create a "public" Directory for Non-Image Content in Base Template

[jmandel]

Currently, non-image content is being placed in the "images" directory to ensure it is included at the root level of the build output. This practice is not semantically clear, as the "images" directory implies it should only contain image files. To improve clarity and organization, a new directory, tentatively named "public," should be created in the base template to hold non-image content.

This change would help developers better understand where to place their files and make the content structure more logical.

Acceptance Criteria:

1. A new directory at input/public is supported in the base template.
2. Non-image content placed in the "public" directory should be accessible at the root level of the build output.
3. Documentation is updated to reflect this new directory and its intended use.

[costateixeira]

Related to HL7/fhir-ig-publisher#780

[lmckenzi]

We will define input/assets which we think is a clearer name than 'public'. It will be handled the same as 'images'. It will be possible to have content in both folders and content of both will be propagated to output unchanged.

[jmandel]

"assets" works fine, thanks! When supporting both images + assets I'd say it should be an error to have overlapping content like:

• images/a
• assets/a

Or

• images/b/one
• assets/b/two