Switch use of relatedItem to the LINKS1 file mechanism

[martindholmes]

We have relatedItem entries in a few files such as EIRE1, but we're not rendering them anywhere. We should switch these over to the LINKS1 mechanism, and then figure out how to render them for primary source transcriptions, which don't get the leading daisy-listing of stuff that born-dig files get, which is where the links are available.

[martindholmes]

Decision 2022-10-19 meeting: Add the Related items/disambiguation and Documents mentioning X links to the top of primary source documents, and then convert the relatedItem tags in EIRE1 over to linkGrps, and check the results.

[martindholmes]

Code to generate the related items in all files added in rev 21366. If this doesn't break anything, the Documents mentioning X link should also be added; then we can check all relatedItem tags to make sure they're reflected in LINKS1, and once they are, delete them.