Fix various docs/homepage accessibility bugs

[riknoll]

This PR fixes a bunch of small accessibility bugs. Each commit is a single fix, so you can also review it one by one if that helps.

Fixes microsoft/pxt-microbit#5495
Fixes microsoft/pxt-arcade#4865
Fixes microsoft/pxt-arcade#4868
Fixes microsoft/pxt-microbit#5466
Fixes microsoft/pxt-arcade#3969
Fixes microsoft/pxt-microbit#5473

Does the following:

1. Adds underlines to links within paragraphs in docs pages
2. Adds a title attribute to simulator frames embedded in docs pages
3. Sets the lang on docs html pages to "en" instead of @locale@
4. Gives the carousel arrows on the homepage an outline when focused
5. Marks landmark roles on the docs pages
6. Adds a gulp pxtrunner shortcut for compiling just pxtrunner (much like gulp webapp)

In regards to the @locale@ change, the backend wasn't patching this with anything (it was returning the empty string) so I'm not sure how this was supposed to work. We also use this on index.html, but it seems like it always returns en regardless of what my browser locale is or what I pass as a query parameter. Rather than debug this, I opted to just hardcode it to en which is what we do in most of our other webapps anyhow and seems to be the only thing the backend ever returns.

[thsparks]

LGTM!