Definition of good documentation

We'd like a definition of what good documentation looks like in the project. This can go under development documentation, and be linked to from the developer guide.

We've got something like this for Javadoc, in the coding conventions document, but we don't have anything for our Markdown documents.

#### Potential inclusions

Heading capitalisation / nesting structure.

Number of columns in a table.

Avoid including too much information inside a single table cell.

Avoid adding too many links that could be broken, consider how to avoid broken links.

Start by reading the existing coding conventions document and developer guide to find the appropriate development-documentation location and linking style. Add a Markdown documentation standard covering the listed heading, table, and link guidance, then link it from the developer guide and verify the documentation links.

Written by the indexing model from the issue text.