Docs: Accessibility tweaks

Only issues left, that affect most pages are:

```
This select element does not have a name available to an accessibility API. Valid names are: label element, title attribute.
```

for

```html
<select onchange="if (this.value) window.location.href=this.value"> <option value="">Navigate ...</select>
```

```
This form field should be labelled in some way. Use the label element (either with a "for" attribute or wrapped around the form field), or "title", "aria-label" or "aria-labelledby" attributes as appropriate.
```

for

```html
<select onchange="if (this.value) window.location.href=this.value"> <option value="">Navigate ...</select>
```

The changes are mostly for WCAG2AA.