Non-unique element IDs in Transactions page

[mickohanlon23]

Several of header element IDs in input/pagecontent/transactions.xml are not unique, causing certain headers to incorrectly link to other parts of the page.

For example, attempting to link to the Register Health Certificate - Message Semantics section (20.3.2) actually links back up to the Submit Health Event - Message Semantics section (20.1.2).