core::string_view

beast should utter `core::string_view` directly instead of using the alias lifted into its namespace, for clarity in documentation and source code. the exposition in the docs should also be changed.

Search the Beast source and documentation for uses of the namespace-lifted string_view alias and identify the corresponding exposition text. Replace those references with core::string_view while preserving the intended meaning. Done means the source and documentation consistently use the explicit name; check the resulting documentation and relevant build or test checks.

Written by the indexing model from the issue text.