Symbols explainer post-workshop

[matatk]

Update the Symbols explainer.

Preview: https://github.com/w3c/adapt/tree/symbols-explainer-post-workshop/explainers/symbols.md

Closes #295

[matatk]

Per the call, we want a hyperlink in the intro that demonstrates that people have been using the Bliss language for some time.

[matatk]

Some other notes from the call:

• Need to decide whether the section on the registry should be part of this explainer or not - the plan was not but maybe it should be included? There is contradiction in the explainer as it is now.

  • Agreed by the group on 2025-08-05

• Need to rename the explainer, as there's no 'Symbols Module' anymore.

[matatk]

I've made the couple of final tweaks we agreed, so I think it's time to mark this as ready for review, and merge it soon. The registry section does need better integration into the rest of the doc, and we have various other parts highlighted for further work, but merging this provides us with a good basis for those more focused additions/changes, which we could do in separate PRs.