Update docs/guides to reference MFA

[iansltx]

For #23910; docs for #22078.

Also fixes "does Fleet supported" typo.

[iansltx]

@rachaelshaw I thought about mentioning emails as a soft dependency on the reference architecture page, using somewhat similar language to what I added in the email backends area of server config. LMK if you want me to try my hand at that, or if you want to back out the server config mods feel free to revert that bit.

I didn't see any articles specifically talking about auth that weren't in the context of SSO, so the changes here wind up being minimal.

Swapped the MFA link in the pricing table as 22078 feels a bit more well-groomed for explaining what got implemented than the original request. Feel free to revert that if I made the wrong decision.

See also: this idea in #g-demand

[iansltx]

@rachaelshaw Fixed the typo I mentioned. Should be good to go here?