fix tab navigation in side docs

[riknoll]

fixes microsoft/pxt-microbit#6330

this pr fixes a few side docs tab navigation bugs

• the "go back" button wasn't tab navigable. it now has a tabindex and was converted to an <a> element
• the pop-out button was at the end of the tab order in the side docs. it's now the first element in the order
• the rendered SVG blocks still had the tabindex attribute on some elements

[srietkerk]

What pop-out button are you referring to? Is it this one?

It's first in the tab order for me in beta and live when I open the help docs. Or did adding the a tag mess with things?

[riknoll]

@srietkerk no, the one that appears in the top-right of the side docs and opens the page in a new tab