Re-organize page docs

ASAN works on Ruby 3.4 and later, so we can remove the point about using
master branch for ASAN.

  I had issues building Ruby as I was using git 2.30.
  The error was:

  > file2lastrev.rb: git rev-parse failed error: could not expand
  > include path '~/.gitcinclude'

  The default system git config includes paths making use of `$HOME`,
  but that env variable gets removed when building: https://github.com/ruby/ruby/blob/7070b1b1968f31f999636b42a1af872458cd5a81/tool/lib/vcs.rb#L546-L547

  It works with git `>= 2.32` thanks to the `GIT_CONFIG_SYSTEM`
  feature which gets set to a Null IO in order to override
  the system gitconfig. https://github.com/ruby/ruby/blob/7070b1b1968f31f999636b42a1af872458cd5a81/tool/lib/vcs.rb#L549

* 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