minor #16320 [Security] Merge standalone Security component docs with Security guide (wouterj)

This PR was merged into the 5.3 branch.

Discussion
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[Security] Merge standalone Security component docs with Security guide

With the new system, the Security component docs are outdated/unusable.

Most of the component is not really practical to use outside the Symfony framework anyways (e.g. you need to set-up the HttpKernel to use the Security HTTP component as designed). So it's not worth maintaining documentation for the few (if any) people doing this. They are operating on such an advanced Symfony level that they'll be able to make sense of the component themselves.

The README of the Security Core component will be updated to showcase the basic usage: https://github.com/symfony/symfony/pull/447780

I'm targeting 5.3 to be in sync with all other Security rewrites done in the documentation.

Commits
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94bd8d2 Merge standalone Security component docs with Security guide

Author: javiereguiluz