Accessibility fixes [a11y]

- See, this bug/issue: https://github.com/nfreear/nfreear.github.io/issues/1

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- [ ] Relative font-size for main text;
- [ ] Relative font-size for headings, etc.
- [x] Add [WAI-ARIA](http://w3.org/TR/wai-aria/) landmark roles;
- [x] Control `menu-toggle` via keyboard (WAI-ARIA/ Javascript) – narrow screens;
- [ ] ... ?
