Add documentation to show usage of multiple containers

[savitaashture]

Knative Serving release v0.16.0 has support for multiple containers as an alpha feature.

To propose multi container feature to beta we should have a documentation to show the usage of multiple containers as part of knative service yaml.

Describe the change you'd like to see
A clear and concise description of what you want to happen.
The documentation should have a description of

1. Multiple containers usecases
2. How to make use of feature flag if someone want to use just one conatiner
3. What's the status of feature in alpha and beta
4. How to differentiate main container from sidecar container
5. What are all the allowed options as part of sidecar containers
6. Providing sample example details -> can be referred from this PR [multi-container]Add samples for multi container support #2732
7. Support from CLI(kn) -> can be find details from this Add CLI support to specify multiple containers client#997

/cc @markusthoemmes @dprotaso

[savitaashture]

serving issue for more info knative/serving#8664

Design Doc : https://docs.google.com/document/d/1XjIRnOGaq9UGllkZgYXQHuTQmhbECNAOk6TT6RNfJMw/edit?ts=5e25d093#heading=h.n8a530nnrb

[knative-prow-robot]

@savitaashture: The label(s) kind/feature cannot be applied, because the repository doesn't have them

In response to this:

/kind feature

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