Inline icons in text for Godot documentation

[shane-smith]

Your Godot version: N/A

Issue description: It would be helpful to be able to represent Godot icons in the documentation text. This was suggested by @Calinou in a review comment on #8460 so I am creating it here as a separate issue.

URL to the documentation page (if already existing): On this page, one of the buttons is currently represented as "A+" - but after the PR above is merged it will be represented as "A>" instead. Neither of these are ideal, as they don't reflect the actual icon being used:

@Calinou mentioned: "I feel writing the icon as text isn't ideal as it can end up being confusing. We should probably look into introducing inline icons in text in a future PR."

This issue is a request for that to be possible.