Document tools and clean up src/utils directory

[wendellpiez]

User Story:

As an OSCAL user and developer, I need to be able to find and run OSCAL tooling.

Since we are refactoring the repositories for SOC, these are now in at least one other repository, namely usnistgov/oscal-tools.

Specifically, several subdirectories in the src/utils/util directory of this repository are being moved to the oscal-tools repository. (WIP now here: https://github.com/wendellpiez/oscal-tools/tree/xslt-setup - there will be a PR.)

Accordingly, these folders, at least, can be removed from the OSCAL repo: author, lib, publish, convert.

Additionally, the website needs some enhancement to point OSCAL users/devs to the tools repository and to its documentation.

Goals:

1. Remove code no longer being maintained in this repository
2. Enhance the OSCAL web site with guidance on how and where to find tools

Dependencies:

usnistgov/oscal-tools#4 represents the other end of this work, and should be completed before the web site is updated.

Acceptance Criteria

• All OSCAL website and readme documentation affected by the changes in this issue have been updated. Changes to the OSCAL website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

[wendellpiez]

It appears this was done in #970

[wendellpiez]

The work is basically done but there is at least one error in the readme most affected.