Add a new-tabbed variant for the Replite directive, and allow customisation of its button text

[agriyakhetarpal]

Description

This continues #165, #223, and #227 for the Replite directive. We do not subclass from the BaseNotebookTab class here since we need to construct a URL with quoted code that has been passed to the directive (similar to the TryExamples directive).

I was initially unsure about doing this as I mentioned in #227 (comment), but the implementation seems to be in line enough with that for other directives and felt straightforward to me. This PR shall bring feature parity for all four major directives, and all five directives can now have new-tabbed variants. As a plus, customisation is allowed for this directive's new-tab button text as well, either by a global replite_button_text value or on a per-directive basis.

Changes made

• A new RepliteTab class that inherits from docutils.nodes.Element. Here, we construct a URL for the REPL based on the input; and we return it when the :new_tab: option is enabled; otherwise, we return RepliteIframe instead.
• Documentation updated for the global replite_button_text option for conf.py, and a sample directive added to showcase Replite opening in a new tab.

[agriyakhetarpal]

I've opened yet another PR today where your review would be appreciated, @Carreau 😁 Thanks a lot!