guidelines.xsl generation update

[sydb]

I got caught once again by the fact that P5/Utilities/guidelines.xsl is a generated file. (And in front of my mentee, making it even more embarrassing.) So the main point of this PR is to make sure there is a very very obvious warning in that file that says it is a derived file. But while I was at it, I converted the generation mechanism to XSLT.

Various changes include:

• Use new XSLT pgm guidelines_model_2_executable.xslt instead of using Perl (to change paths to imported files)
• Add XML declaration to output
• Allow XSLT processor to indent output (but not for guidelines-latex, as I do not know enough LaTeX to know that is OK)
• Use XSLT 3
• Add warning comments in derived output
• Correct lots of mistyped "UTF-8"s

[sydb]

BTW, I have run make clean validate html-web test exemplars teiwebsiteguidelines pdf epub on this branch in a Docker environment, and it all worked.

[raffazizzi]

F2F@Guelph subgroup: this looks good and @joeytakeda can go ahead and merge