[Docs] Add docs for new `Rotate uninstall token` feature

https://github.com/elastic/kibana/pull/278923 Adds the ability for a user to rotate the uninstall token on a tamper protected agent policy, which previously was generated and then could not be changed. Currently the docs at https://www.elastic.co/docs/solutions/security/configure-elastic-defend/prevent-elastic-agent-uninstallation explain that "One unique uninstall token is generated per Elastic Agent policy", which is true, but it should be added that it can now be rotated in case it is leaked or for any other reason. Today (before this change), the only recourse was creating a new policy and assigning all agents to the new policy to get a new uninstall token, this changes that.