Appsody.dev Development guide should include how to add links to Markdown

[helenmasters]

We don't currently have an example of how contributors should add links to other docs on the website into the Appsody Markdown files. We should add this into our Development guide as there is currently confusion over whether the links should be "/content/docs/stacks/package.md" or "docs/stacks/package.md" or "docs/stacks/package"

David, please can you resolve this discussion and add the relevant example into the Development guide.

As part of this work please can you also include a comment about which directories within the website you can 'short link' in this way (ie does it only apply to docs/...) as there is a reference to the contributing guide in the stack documentation which has a fully qualified link which is confusing as that doc is in GitHub website repo