Document Markdown syntax

Created by: siddharthab

Feature request description

Markdown rendering requirements (syntax support and styling) can vary between organizations. It will be nice for site admins to be able to plug in their markdown engines.

Is your feature request related to a problem? If so, please describe.

The current markdown rendering engine is a black box to users, and it is difficult to guide users into how to configure their Markdown editors such that they support the same syntax as what Sourcegraph uses. Even if we know it's blackfriday, for instance, it is not entirely clear which options are enabled.

Describe alternatives you've considered.

Asking users to import their commits into Sourcegraph to check their rendering.

Additional context

None.