Explain how to link multiple voting keys

[segfaultxavi]

As hinted here:
https://docs.symbolplatform.com/serialization/coresystem.html#voting-key-link

Up to 3 voting keys can be linked, so when one expires you can have another one ready while renewing the old one.

This process needs to be explained in detail somewhere. More precisely, HOW to achieve this:

1. Notes about voting key lifetime and how often node admins need to upgrade
2. Multi-file support to the manual nodes. How they can create a second file and link it.
3. Once the bootstrap PR is approved and merged, how node admins will need to upgrade the nodes to get new/future voting keys:
   • bootstrap --upgrade upgrades configuration, compose, etc. It doesn't upgrade the voting key by default
   • bootstrap upgradeVotingKeys upgrades the voting keys adding a new one when required. This command does all the math for the user. When to create, the start and end epoch. Same with the link command, it knows when and how to link/unlink
   • Once bootstrap created a voting key, it needs to be linked. This can be done using the link command or symbol-cli.
   • So the process for users is: Every 6 months run upgradeVotingKeys followed by link. Independently, run --upgrade whenever there's a new version of the server.

More details:
https://nemgroup-workspace.slack.com/archives/C01T1VD5A3D/p1622657108128900
https://nem-software.atlassian.net/wiki/spaces/TD/pages/661553206/Voting+Key+Files