Add GRCh38 custom-track example files

[twalsh-ebi]

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Description

This PR would add GRCh38 custom-track example files, and would link to them from relevant pages.

It would leave unchanged the existing custom-track example files, as well as relevant test config files.

As an additional change, it would fix a broken link to Jensembl API docs.

Views affected

This would update three example file links in the webpage on Adding Custom Tracks to Ensembl.

Possible complications

None expected, since existing example files would be unchanged.

Merge conflicts

None detected.

Related JIRA Issues (EBI developers only)

N/A