Extend the construction/current/index.md to cover all known GitHub Markdown features

[orcmid]

The test file needs to be extended to more cases.

1. additional text decorations
2. Links and cross-references on a page, between pages on a site, and externally - outbound and inbound.
3. Decoration of links with regard to accessibility and other cases.
4. special paragraph forms
5. Images
6. code blocks
7. formulas and ways to have them rendered

Then there needs to be consideration of how pandemic Markdown is handled or not.