Tidy up documentation for public consumption

[tobystokes]

To make it clear to (hopefully still only casual) browsers the purpose and usage fo this system.

[tobystokes]

@argenteum you could add relevant accessibility and browser standards that you think all SMG projects should adhere to in https://thesciencemuseum.github.io/web-design-system/docs/standards.html ?

(And for the docs, it's probably okay to just edit the markdown in e.g. https://github.com/TheScienceMuseum/web-design-system/blob/master/fractal/docs/07-standards.md without needing to run and build the whole system, especially if you have an editor that can do markdown preview)

[argenteum]

@argenteum you could add relevant accessibility and browser standards that you think all SMG projects should adhere to in https://thesciencemuseum.github.io/web-design-system/docs/standards.html

Will do