DOC-2994: Connectivity Section - Transit Gateway branch

[cmilesb]

Staging link: https://docs.redis.com/staging/DOC-2994-tgw/subscriptions/connectivity

Similar to #2908, but with the Transit Gateway page moved.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
5 out of 6 committers have signed the CLA.

:white_check_mark: cmilesb
:white_check_mark: amberdoctor-redis
:white_check_mark: LiorKogan
:white_check_mark: ggeorges2015
:white_check_mark: rrelledge
:x: NoamSternRedis
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[joniredis]

Correct preview link

[cmilesb]

Holding off on this until the site merge.