docs: Add paragraph about @ operator

[Secrus]

Resolves: #5136
Resolves: #3142

Added documentation on @ operator in dependency specification.

[github-actions]

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[Secrus]

@clintonroy I have addressed your comments.

[neersighted]

I'm wondering if this is really the correct place to add the docs -- since there's no surrounding mention of poetry add it feels out of place. If this really is the best place then the opening sentence/section title should probably be something along the lines of "adding packages with constraints using the CLI" -- if there's a more natural location with other CLI docs, we should just add a link to that section.

[Secrus]

@neersighted I added link in add command docs and changed first paragraph of the @ documentation.

[github-actions]

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