* Clarify “Building Ruby” docs

* Fix test examples to work from `build` dir

* Clarify “Testing Ruby” examples with real examples

All the commands should run correctly by default, without the user needing to modify them. This builds confidence that the relative paths are working correct from within the `build` directory.

Also, let’s use a consistent example throughout, for greater clarity.

* Improve examples to use `-v` flag in-context

This shows the correct way to combine `-v` with another parameter, e.g. a specific file to test.

* Other readability improvements

* Clarify `make` implementation support

* Fix WASM bullet/code indentation

* Use `console` code highlighting where appropriate

… which handles the prefix `$` correctly.

* Migrate feature proposal template to MarkDown

* Set language on code blocks

- `OpenSSL` here is not the class.
- libexecinfo on FreeBSD is present in the base system of all
  supported releases, since 2017.

Get rid of using ambiguous demonstratives as link anchors, for the
accessibility's sake.

Clang 18 has the bugfix we need, so we no longer need to tell people to
compile Clang from source.