DOC Add info for links to docs and images for modules

[emteknetnz]

Issue silverstripe/.github#420

[GuySartorelli]

Suggested change

	When updating documentation for [`silverstripe/developer-docs`](https://github.com/silverstripe/developer-docs/), which is the content repository for [`docs.silverstripe.org`](https://docs.silverstripe.org), links to other pages in the documentation should use the URL slug, not the file name.
	When updating documentation for [`silverstripe/developer-docs`](https://github.com/silverstripe/developer-docs/), which is the content repository for [`docs.silverstripe.org`](https://docs.silverstripe.org), links to other pages in the documentation should use the URL slug (e.g. `/contributing/documentation/`), not the file name.

[GuySartorelli]

Suggested change

When updating docs that are in a modules `docs/en` folder, and will be shown in the "Optional features" section of `docs.silverstripe.org`, and are also intended to be readible on GitHub, then links to other pages should use a relative path to the file name, not the URL slug.

When updating docs that are in a module's `docs/en/` folder, and will be shown in the "Optional features" section of `docs.silverstripe.org`, and are also intended to be readible on GitHub, then links to other pages within that module should use a relative path to the file name, not the URL slug. Links to documentation outside that module should use a full URL without the version number (e.g. `https://docs.silverstripe.org/en/contributing/documentation/`). API doc links should still be used in the format described below.

[GuySartorelli]

Also, put this note below the bits about "Relative documentation links" and "Absolute documentation links" below which are specific to docs in silverstripe/developer-docs.

[GuySartorelli]

Should target 5.4 or 6.0 so it doesn't have to wait til the next beta to be visible.