Improve overall new examples' accessibility

[ffoodd]

Small tweaks caught by Pa11y on Boosted. Mostly:

1. ensuring having main and h1 on each page,
2. a few input missing a label,
3. some redundant alt on img inside a link,
4. contents not wrapped in a landmark.

I ignore some other errors since they're not applicable for such snippets IMHO (several banner roles instead of one, for example).

Non-whitespace diff: https://github.com/twbs/bootstrap/pull/33772/files?w=1

[patrickhlauke]

Note that my requested changes (the addition of aria-current) are admittedly snuck in here - they're not new things added/done by this PR, but as I stumbled across them...would be good to sneak those in too. other than that this PR looks good to me.