Indicate alternatives for deprecated commands

[stockholmux]

There are a few issues on deprecated commands (#304, #260, #290) and we've not done a good job about describing how deprecation works in Valkey broadly (functionally command deprecation means there is a better command to use in place of the deprecated command) or specifically (which command to use instead).

These alternatives should show up both in the command reference list (https://valkey.io/commands/) and on the individual command pages (https://valkey.io/commands/getset/).

[zuiderkwast]

I believe the REPLACED BY is in the JSON files in the code repo. I think we may be missing this in the zola logic.

[zuiderkwast]

Example: https://github.com/valkey-io/valkey/blob/unstable/src/commands/setex.json#L10

[stockholmux]

@zuiderkwast Yeah, valkey-io/valkey-io.github.io#286 uses replaced-by from the JSON.

I guess two more things before we close out this issue:

1. That PR only covers the command pages, we need to implement it on the command list.
2. What about man pages - are you covered there? (Is it needed?)

[zuiderkwast]

Yes I beliece man pages are covered enough. There is a generated command listing page valkey-commands(7). It inserts "Deprecated" next to the command name and description. On each command page, IIRC we also insert the replaced-by somewhere on the page.