Move hardware interface README content to sphinx documentation (backport #1342)

[mergify]

This is an automatic backport of pull request #1342 done by Mergify.

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[christophfroehlich]

@mateusmenezes95 could you please check if this is feasible for humble, too?

[mateusmenezes95]

@mateusmenezes95 could you please check if this is feasible for humble, too?

@christophfroehlich could you, please, give me hints on how to check this?

[christophfroehlich]

@mateusmenezes95 could you please check if this is feasible for humble, too?

@christophfroehlich could you, please, give me hints on how to check this?

just a quick check if the code snippets make sense for humble (has the API of the used methods changed?)

[mateusmenezes95]

@christophfroehlich I compared the snippets in the documentation with the example 1 of ros2_control_demos and there is no API change of the methods used in the examples. Hence, I think we can proceed with the merge in the humble branch.