Fix "Error reading page 'index.md': extendMarkdown()"

[iamEAP]

What / Why

Saw reports in discord of folks who merely run pip install mkdocs-techdocs-core==1.* in their set up, then run mkdocs build, get the following error:

ERROR    -  Error reading page 'index.md': extendMarkdown() missing 1 required positional argument: 'md_globals'
mkdocs failed, exit code: 1

This PR:

• Adds a simple end-to-end test (run mkdocs build after freshly installing this plugin) to catch run-time errors like this.
• Tightens the Markdown version constraint to only those versions that are compatible with the current version of mkdocs.

Follow-up to #74.

See also: mkdocs/mkdocs#2892