Maintain the list of reference documentation authors in a single place

[wilkinsona]

The authors are currently listed in multiple places:

• index.htmladoc, index.htmlsingleadoc, and spring-boot-reference.pdfadoc in 2.2.x
• index.adoc, and index.singleadoc in master

To guarantee that the list of authors is the same across the HTML, HTML single, and PDF documentation, we should try to list the authors in a separate file that is included in each index file.

[cicioflaviu]

Can I take this task?

[wilkinsona]

Thanks for the offer, @cicioflaviu, but I think we should figure out what's going on with #20934 first.

[wilkinsona]

While researching #20934, I learned that multiple authors should be separated by semi-colons and not commas as we currently do. We can take care of that at the same time as switching to a single source for the list of authors.