Remove free/paid matrix from FAQ; add (out-of-band) promo for support from Tetrate

[oletizi]

Per Feb 2, 2021 with Prasad:

• remove the free/paid matrix from FAQ
• everything on GetIstio is always free
• Need to add a promo for support from Tetrate to the FAQ and the homepage that links to https://tetrate.io/getistio

[oletizi]

Submitted PR: #64

[lunchboxav]

For the last point, a promo banner in homepage that links to tetrate.io/getistio is available right on top of footer. The link is fixed in PR #85.

[lunchboxav]

@oletizi can we close this down?