docs: update docs to capture Permissionless ICS (backport #2289)

[mergify]

Description

This is a first attempt at updating the docs to capture the latest Permissionless ICS changes.
This PR does not yet change the docs under:

• docs/docs/validators/changeover-procedure.md
• docs/docs/validators/joining-neutron.md
• docs/docs/validators/joining-stride.md

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• included the correct docs: prefix in the PR title
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• Checked that the documentation website can be built and deployed successfully (run make build-docs)

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This is an automatic backport of pull request #2289 done by [Mergify](https://mergify.com).