External link checking with HtmlProofer errors when creating new pages

[mrwilson]

If external link checking is enabled with HtmlProofer, the next page created will cause the link checker to fail when using auto-generated links to the page source.

This is because the page source link does not yet exist in source control, but does exist in the branch, so the HtmlProofer fails.

We patched this in di-authentication-tech-docs by ignoring any links related to the source repo url.

https://github.com/alphagov/di-authentication-tech-docs/blob/main/config.rb#L24

[PippaClarkGDS]

Action for this: we would need the consideration to add it as an optional extra - something that can be turned on in the config. Possibly bundle it with the tech docs gem with some sensible defaults, have a flag to turn it on, and another flag to override the configuration.

[lfdebrux]

Related to #186, where we also look at putting html-proofer in the template.