Add support for blockquote and table alignment in HTML <-> Markdown mapping

[david-waltermire]

User Story:

As an OSCAL content developer, I need to be able to use <blockquote> in html and the equivalent > in markdown. I also need table alignment markup in Markdown :--- (left align) and ---: (right align) to be mapped to use of ` in HTML.

Goals:

• Add support for <blockquote> in html and the equivalent > in markdown to the metaschema to the complex-markup data type.
• Add support for table alignment markup in Markdown :--- (left align) and ---: (right align) to be mapped to use of <td align="left|right"> in HTML for the complex-markup data type.
• Update prose documentation to include these features.

Dependencies:

Uses the type system added in issue usnistgov/OSCAL#436.

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.

[david-waltermire]

We should follow the CommonMark blockquote specification.

We may want to implement less than what CommonMark allows as well to make this easier to achieve.

[wendellpiez]

This Issue depends on a complete spec of the current Metaschema-supported Markdown subset, with which it needs to be aligned.

Can we have a flag for Issues related to Markdown?