feat #1154: use semantically more meaningful tags

[HerrBertling]

Summary

See #1154 for info – this replaces some divs with header/main/aside tags and adds an aria landmark to the search

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No (not that I am aware of, at least)

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

Merging this PR closes #1154

[HerrBertling]

Just checked in IE – forgot to set display: block for the main element. Additional commit incoming!

^{Also, I should get my author mail address & name in order between work and private machine 🙈}

[ulivz]

That's great, thanks!

[nelson6e65]

There is a typo introduced by this pr. Please check fix in #1181