Create doc on upgrading CCF

[letmaik]

The doc should contain things like:

• If upgrading to a new major version, make sure to first upgrade to the latest current patch version and do a release. See also https://microsoft.github.io/CCF/main/build_apps/release_policy.html#operations-compatibility.
• Update files xyz and create a PR.
• Read the CCF changelog and, if needed, create issues for any outstanding tasks or newly available features that are useful here.

NOTE: This could be done as an issue template like https://github.com/microsoft/scitt-ccf-ledger/pull/57

[achamayou]

I don't think there is a lot to do here, other than read the changelog, because:

1. GH Copilot has gotten quite good at minor updates
2. More disruptive updates like 6 -> 7 require manual care, because binaries are merged across targets etc. There is no a single playbook to follow.