Restructure docs for KTX

[samtstern]

Main changes:

• Move the main page for this repo on firebaseopensource.com to be docs/README.md so we can present a less-technical overview.
• Add a new folder docs with subpages
• Add links to the main and KTX reference docs

cc @ashwinraghav @vkryachko we will need to actually fill out the content on KTX

[earnestsaunders]

cc-ing me

[vkryachko]

Overall LGTM(modulo actual ktx content)

Is there a way for us to see what it would render like? Curious how tabs work

[samtstern]

@vkryachko yeah I need to add better staging capabilities, but here's a repo that has tabs:
https://firebaseopensource.com/projects/firebase/emberfire/

You can see them in the blue bar, they're the series of external links.

[samtstern]

@vkryachko @earnestsaunders can you take a look at the content I put in? It's super basic at the moment.

[vkryachko]

LGTM 👍

[samtstern]

Local preview looking good!

[google-oss-bot]

@samtstern: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
connected-check-changed	0a4bf72	link	/test connected-check-changed

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