[DOXIA-713] Document limitations of the parsers/sinks

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[DOXIA-713](https://issues.apache.org/jira/browse/DOXIA-713?redirect=false)** and commented

Due to the Sink API abstractions some language features of the underlying markup languages or the Sink API are not supported for parsers and/or sinks. Those should be documented at https://maven.apache.org/doxia/references/index.html.
Examples are:
- markdown sink: anchor
- markdown sink: figure (only via HTML markup)
- markdown sink: table justification
.....

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No further details from [DOXIA-713](https://issues.apache.org/jira/browse/DOXIA-713?redirect=false)

Start at the Doxia references page linked in the issue and review the parser and sink behavior, beginning with the Markdown sink examples listed: anchors, figures, and table justification. Done means the relevant unsupported or limited language features are documented clearly on that page, including any additional limitations found during the review.

Written by the indexing model from the issue text.