Revise rolling upgrade guide

[hammerhead]

About

We received feedback that the rolling upgrade guide was hard to follow for new users who aren't familiar yet with distributed systems. While the overall upgrade process was documented well before, there were the following issues:

• Step-by-step instructions were interrupted by lengthy educational background information on certain parameters and their implications, causing a loss of focus. Now: Focus on the flow with default settings, elaborate on possible deviations separately.
• Added an initial health check to ensure the cluster is green before starting
• Removed a warning that upgrades aren't possible from "testing versions". This is not applicable if we understand "testing versions" as the testing channel, which are regular releases. It may have targeted nightly builds, but those aren't supported anyway.
• Some of the SQL statements were outdated, querying _node instead of node from sys.shards, and also unnecessarily limiting their result to the schemas doc and blob.
• There was an incorrect statement about writes only succeeding if a quorum of shards are present, contradicting the documentation of write.wait-for-active-shards
• Replaced the outdated system command with modern systemctl in examples
• The "Repeat" step was inconsistent, asking to issue SET GLOBAL TRANSIENT "cluster.routing.allocation.enable" = 'new_primaries'; after upgrading every single node, even though it wasn't reset
• Added a post-upgrade health check to see if everything is back to green
• Provided clearer guidance when to use min_availability options full, none, or primaries

Pending clarifications

• Unsafe cluster.routing.allocation.enable recommendation for zero downtime upgrades crate#19751

Preview

https://cratedb-guide--599.org.readthedocs.build/admin/upgrade/rolling.html

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