docs: Add Dragonfly as an online-store option to Feast Readme and docs

[yirenlu92]

What this PR does / why we need it:
Adds Dragonfly (drop in Redis replacement as an online-store option)

Which issue(s) this PR fixes:

Fixes #

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[worldsoup]

hey @adchia just wanted to ping you on this...any changes needed to get this docs update merged? Thanks in advance

[adchia]

Hey! Sorry for the delay. Looks like you manually modified the README, but actually you should be using make build-templates to generate the README (+ changing https://github.com/feast-dev/feast/blob/master/infra/templates/README.md.jinja2)

[adchia]

Fixed it for you.

/lgtm

[Niennienzz]

Hi @adchia, thank you for reviewing!

I am wondering when/how will the Dragonfly page actually land in the Feast doc?

[adchia]

i'll cut a release soon and then it'll show up!

[Niennienzz]

i'll cut a release soon and then it'll show up!

Got it, thank you sooo much!