docs: replace dead communityinviter link with Envoy maintained redirect

[bekerr1]

Commit Message:

docs: replace dead communityinviter link with Envoy maintained redirect

Additional Description:

Made sure there is nowhere else that references the communityinviter service

$ grep -rE "communityinvite" ./* 
$ 

Interestingly, mobile had the right link already

$ grep -r "envoyproxy.io/slack" ./*
./mobile/README.md:  [here](https://envoyproxy.io/slack). We can be found in the **#envoy-mobile** room.
./README.md:* [Slack](https://envoyproxy.slack.com/): Slack, to get invited go [here](https://www.envoyproxy.io/slack).

Risk Level: Low

Fixes #46347

[repokitteh-read-only]

Docs for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-cncf-pr/46508/docs/index.html

The docs are (re-)rendered each time the CI Envoy/Prechecks (docs) job completes.

🐱

Caused by: #46508 was opened by bekerr1.

see: more, trace.

[repokitteh-read-only]

Hi @bekerr1, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #46508 was opened by bekerr1.

see: more, trace.