Move docs/README.md into docs/docs/ so mkdocs can include it

[CapnKernel]

In discussions about previous changes I'd made to the README.md for docs, we noted that the page wasn't currently reachable from the main docs page, and perhaps we could do that. Well here it is.

With these changes, the README.md file previously in the top level docs dir has been moved one level deeper to docs/docs/documentation-howto.md. I've moved it deeper so mkdocs can see it. (Is there another way that doesn't involve moving it down?) Also, we can't call the moved file docs/docs/README.md, because mkdocs says it clashes with docs/docs/index.md.

There are three commits.

1. Already sent in Improvements to the documentation instructions #11044.
2. Move docs/README.md to docs/docs/documentation-howto.md with no changes.
3. Changed examples to stop mkdocs interpreting {{ image() }} and {{ icon() }}, and added documentation-howto.md to mkdocs.yaml.

I suggest reviewing them as commits, rather than the end-to-end changes.

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[SchrodingersGat]

@CapnKernel please address merge conflicts here.

At the top of this page, the wording needs some tweaking now that it is included into the documentation itself:

Maybe instead of "this repository hosts..." it should be something like "This page describes..."