Docs: Unified envoy gateway installation documentation.

[yuluo-yx]

Description:

In the Quick Start section of the website, there are already steps on how to install the envoy gateway.

https://gateway.envoyproxy.io/v0.6.0/user/quickstart/#installation

In other eg-related articles, the installation section of the Quick Start is referenced.

In the individual articles, the steps for installing eg are rewritten, but the contents are the same.

e.g.:

1. https://gateway.envoyproxy.io/v0.6.0/user/http-routing/
2. https://gateway.envoyproxy.io/v0.6.0/user/grpc-routing/
3. https://gateway.envoyproxy.io/v0.6.0/user/tcp-routing/

IMO, it's possible to unify them and reduce duplicates in the document, if this is something that needs to be done, please assign it to me and I'll complete it

[shawnh2]

thanks for finding this, you can dedup these contents under the latest doc