`report_issue_url` cannot be configured in `config/tech-docs.yaml`

[lfdebrux]

Currently the tech docs documentation tells users that they can configure the url for the 'report problem' link by adding some configuration to config/tech-docs.yaml [1]. Unfortunately, a bug in the implementation of this feature means that the configuration actually has to be added to config.rb:

tech-docs-gem/lib/govuk_tech_docs/contribution_banner.rb

Line 22 in 66f7117

url = config[:source_urls]&.[](:report_issue_url)

Line 22 should actually read source_urls from the tech_docs hash.

What should change

We should change it so that the configuration is read from config/tech-docs.yaml, while retaining a fallback ability to configure from config.rb as before, to avoid a breaking change.

User need

As a user I need to be able to use example code from the documentation.