[Accessibility] Discussion: <main> and <section> tag structure

[aphelionz]

For ease of screen reader navigation, we should talk about transitioning the page structure to use <main> tags as well as <section> tags.

[timroes]

This discussion might be now linked to #11516 .

I would in general love to have a proper main, section structure. It might also get us rid of any skip links and would still leave us flexible enough, to initially focus within the page somewhere (e.g. the query bar), without having larger problems with the skipping links (since they might be more likely at the beginning of the page).

I think we should look at each app individually and sketch a grid, for that page to determine each sections (like @aphelionz did in #12633) and maybe also other landmark roles, like search, complementary (if we at all want to use it) and the appropriate labels for those sections.

[chrisronline]

This feels like a larger request. Let's move to phase 2.