[Enhancement]: Documentation should be published, and it should belong in the gem

[JC-DSIT]

What should change?

This issue links up PR #83

User need

There needs to be documentation that users can access to understand how to use the tech-docs-gem and the tech-docs-template. Since the original documentation site was closed down it is not publicly available.

It has currently been published via GitHub Pages on the existing repo, but would be better moved to this gem to simplify maintenance and make it easier to find

Proposed solution

Move the content to here, and then publish from here

[JC-DSIT]

@huwd and @NathanD-GDS - I think we could get this done in a matter of minutes if we just copied the source files of the existing docs and used them as the example site in the gem and published from there.
Then we reduce one step in the process, and archive the old repo / redirect it to this one. Also means we can more easily test and document updates and changes