Docs: rewrite the docs/README file
Docs: rewrite the docs/README file

I originally wrote this file when we first added snippets testing
and a lot has changed. We've grown quite fond of the
`// TESTRESPONSE[s/foo/bar/]` construct, for example, but the docs
discouraged its use.

Relates to #18160

strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc