Adding the Header component to primer

[jonrohan]

This ports the recently created Header component from .com and adds documentation on it.

TODO:

• Include octicon css #857 Include the @primer/octicons css in the build process. There's style defaults `fill: currentColor;display: inline-block;vertical-align: text-bottom" that will fix a bug in the header example in the docs.

/cc @primer/ds-core

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[shawnbot]

The canonical example includes some unnecessary v-align-middle funkiness that I think we could eliminate with flexbox. I hacked this in the live example and it seems to work well:

Suggested changes apply the fix, but you'll probably want to test it more thoroughly.

[jonrohan]

The canonical example includes some unnecessary v-align-middle funkiness that I think we could eliminate with flexbox. I hacked this in the live example and it seems to work well:

I'll update to use the flexbox utilities, but want to steer clear of adding to the link class, worried it will break a lot of things.

[shawnbot]

This is great, let's ship it! 🚀