Documentation how fill old_signing_keys after changing homeserver.signing.key

This issue has been migrated from [#5958](https://github.com/matrix-org/synapse/issues/5958).

---

Please add documentation with examples how to fill `old_signing_keys` values, when we have new `homeserver.signing.key` and old one.

In `homeserver.yaml` file there are description:
```
# The keys that the server used to sign messages with but won't use
# to sign new messages. E.g. it has lost its private key
#
#old_signing_keys:
# "ed25519:auto":
# # Base64 encoded public key
# key: "The public part of your old signing key."
# # Millisecond POSIX timestamp when the key expired.
# expired_ts: 123456789123
```
So users must place in this file only **public part of your old signing key**, but there are no description how to generate public part, having full old key file? Can you please describe how to do this in description text?