Add HTML Documentation API

[4e6]

Pull Request Description

close #1698

PR adds support for the HTML documentation chunks. HTML chunks are stored in the documentation_html column of the suggestions table. New suggestions_order table allows preserving the order of chunks in the source file.

Note. documentation_html probably makes the documentation column containing the raw docstring obsolete. We may remove it in future releases but will keep it for backward compatibility for now.

Changelog:

• add: documentation_html column to suggestions database
• add: suggestions_order table that stores the order of the module-level entries in the source file
• add: JSON API supporting the updates of the suggestions_order table

Important Notes

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the Scala, Java, and Rust style guides.
• All documentation and configuration conforms to the markdown and YAML style guides.
• All code has been tested where possible.