'External docs' feature strips anchor links

[ChrisBAshton]

See https://docs.publishing.service.gov.uk/repos/hmrc-manuals-api.html#manuals-and-decisions

Every link in that list takes you to https://docs.publishing.service.gov.uk/repos/hmrc-manuals-api/extended_documentation.html, but if you look at the original README, each link takes you to a different anchor point on that page.

We should retain the anchor portion of the link when importing docs, not strip it out.