Docs: update docs for src validate command to add new install subcommand

[jdpleiness]

This is a documentation update for the new install subcommand for src validate that will be introduced in sourcegraph/src-cli#921.

• Updated examples to reflect new configuration layout
• Updated references to src validate to be src validate install
• Made warning that features are experimental more visible

Test plan

Run yarn docsite:serve to stand up documentation.

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 1ef9f8b...e9b1bfa.

Notify	File(s)
@sourcegraph/delivery	doc/admin/validation.md

[abeatrix]

LGTM 🔥

[abeatrix]

We should use emoji more 😆

[abeatrix]

What do you think about adding those nice code snippet to the docs as examples 😏? I also like how clean it currently is though 🥴

[jdpleiness]

I am going back and forth on that too. I think for now leave it as is, then we can always add more later.

[github-actions]

❌ Problem: the label i-acknowledge-this-goes-into-the-release is absent.
👉 What to do: we're in the next Sourcegraph release code freeze period. If you are 100% sure your changes should get released or provide no risk to the release, add the label your PR with i-acknowledge-this-goes-into-the-release.