Add automatic deployment for screen reader staging branch

[BenHenning]

Check for duplicates

• I have searched for similar issues before opening a new one.

Problem

#684 is the initial PR introducing work to a new, long-lived branch: add-screen-reader-support. It's important to be able to easily test these changes and, given that they are in a staging branch for the next number of months, one can't simply use the existing GitHub pages link (https://google.github.io/blockly-keyboard-experimentation/) or self-deploy (easily).

Request

@microbit-matt-hillsdon suggested that we introduce a regular CI deployment for the screen reader branch.

This could be as simple as deploying a subdirectory of the GitHub pages site, e.g. https://google.github.io/blockly-keyboard-experimentation/screen-reader, that includes the pre-released changes (possibly linked against tip-of-tree core Blockly).

Alternatives considered

Most other solutions are probably harder, including:

• Deploying outside pages.
• Deploying to a fork (which is what we initially did with https://benhenning.github.io/blockly-keyboard-experimentation/ but then it was automatically overwritten by the same actions that make the main repo work correctly).

Additional context

No response