Add chevron arrows, SVG optimization, link color

[jessicaschilling]

NOTE! Please don't merge until we confirm chevrons look good in situ in file import UI.

Proposing as minor release:

• Adds chevron-style arrow to be used in file import progress UI pattern: stroke_small_arrow.svg and glyph_small_arrow.svg
• Optimizes all icons for file size
• Updates gateway link to icon guide doc in readme
• Adds link color, confirmed WCAG AA compliant on white background

Ref https://github.com/ipfs-shipyard/ipfs-webui/issues/1206 for icons Ref https://github.com/ipfs/dir-index-html/pull/29#issuecomment-623767787 for link color

[rafaelramalho19]

LGTM 😄

In the future, I'll try to change the way we include the fonts in the docs since it involves duplicating the number of changes per PR 🤔

[jessicaschilling]

Thanks - holding off on merging this until we know how the chevrons look in progress UI.