Permit MediaWiki-style internal links syntax (`[[concept]]`) in OKF Markdown

How about (a future version of) the OKF spec explicitly permit MediaWiki-style internal links syntax in OKF Markdown?

So basically allow `[[topic/concept]]` instead of full `[Topic's Concept](topic/concept)` as an alternative.

Even though not strictly [CommonMark](https://commonmark.org/)-compliant (which [OKF doesn't require anyway](https://github.com/search?q=repo%3AGoogleCloudPlatform%2Fopen-knowledge-format+CommonMark&type=code)),

this is (very) common nowadays, see for example:

- https://obsidian.md/help/links
- https://www.mediawiki.org/wiki/Help:Links
- https://docs.enola.dev/specs/markdown-magic-link
- https://quartz.jzhao.xyz/features/wikilinks
- https://github.com/orbikm/mkdocs-ezlinks-plugin

Start by locating the OKF Markdown specification and its syntax or validation guidance; no specific file or test is named in the issue. Compare the proposed [[topic/concept]] form with the existing full-link form and the referenced CommonMark constraints, then document the accepted behavior and any required examples or validation updates.

Written by the indexing model from the issue text.