doc: Fix docslint misunderstanding scope of code-blocks

Allow to add JS/CSS files to the specific page when an extension calls
`app.add_js_file()` or `app.add_css_file()` on `html-page-context`
event.

I'm very happy to see that the events in Sphinx have been laid out in order as they have been in this doc.

With that being said the notation I noticed was a bit odd. Some of the items in the list refer to events and as a result,
parenthesis are used to indicate the beginning and end of the parameters that the event takes.

In other items in the list, parenthesis are used for additional context for an event. The combination of some particularly very long lines,
the intermixing of comments, pseudo-code, indentation of items following a `for` or an `if` made this very difficult to read on a mobile device.

As a result, I've added whitespace, fixed a typo, reduced the length of some lines by moving them to a new line, and removed all `()` that were not
present to indicate parameters fed to a function.

To be clear the ambiguous warning for missing-reference :ref:,
this separates the warning to missing-label and missing-caption.

To emit a warning dynamically, this also adds a new event:
`warn-missing-reference` to customize warning messages via event
handlers.