CAMEL-15360: fix HTML validation issues

[zregvart]

This greatly simplifies the footer design and fixes HTML issues
identified by check:html script.

We're now using Unicode full width plus symbol so we don't need the SVG
icons for show/hide menu.

[AemieJ]

@zregvart I found something odd about the checkbox outline so, with the following changes I suggested, the outline is removed.

[zregvart]

@zregvart I found something odd about the checkbox outline so, with the following changes I suggested, the outline is removed.

This is the input showing beneath the content made with :before. I've implemented a different solution in the latest changes. We should not remove outline, it helps folk what navigate using the keyboard. That being said I found no way to have the checkbox focused, in the right place with keyboard navigation and not visible at the same time in the latest changes; so we don't have keyboard focus indication (outline) in the latest changes for the plus sign or for the section headings (dt).

[AemieJ]

@zregvart yes, I observed that it's better just the + is included now within the desktop version (> 1024px) as well so best we include separate id for those labels and change display property as per that.

[zregvart]

@AemieJ I think this is now okay to get merged, thanks for reviewing, can you check the latest version when the preview is built?

[AemieJ]

LGTM.