docs: add runtime add remotes with virtual:__federation__

[frolovsky]

Description

Documentation for working with virtual:__federation__ has been added. It addresses user questions such as "how to load a module at runtime."
#642

Describes part of the changes from #481.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Code of Conduct and follow the Commit Convention guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[tobyscott25]

Amazing addition to the docs, I just implemented this in a project and would've loved to have this in the docs for quick reference!

Just one small suggestion, learning about virtual modules in Vite properly helped me understand how this magic worked, so I've just suggested the addition of a note to the relevant page in documentation 😄

[tobyscott25]

Suggested change

	It is not always possible to define the list of remote applications in advance in `vite.config`. Some applications may load the list of these remotes asynchronously when the user visits the website. In such cases, you can use the `virtual:__federation__` API.
	It is not always possible to define the list of remote applications in advance in `vite.config`. Some applications may load the list of these remotes asynchronously when the user visits the website. In such cases, you can use the `virtual:__federation__` API.
	> Note: This is a virtual module, for a deeper understanding of virtual modules in Vite, see: https://vite.dev/guide/api-plugin#virtual-modules-convention