Simplify symbol examples

[matatk]

As discussed we're simplifying the symbol examples to remove anything that may be construed as translation. This PR also tightens up the parsing rules a bit, and doesn't stop us from having multiple symbols within one attribute value in future.

HTML Previews:

• Before: https://w3c.github.io/personalization-semantics/content/#symbol-explanation
• After: http://raw.githack.com/w3c/personalization-semantics/simplify-symbol-examples/content/index.html#symbol-explanation (also the examples section, below)

Questions:

I have some questions about this; the editorial question is: have I marked up "the plus sign (+)" part inkeeping with W3C house style?

For the other questions about this, I've started a thread on the mailing list: https://lists.w3.org/Archives/Public/public-personalization-tf/2022May/0005.html

This may clarify some matters relating to #203.