iscsi improvements

this article can be improved by doing more explanations of the general ideas, terms, and then diving into concrete and simple examples

- change title to iSCSI initiator (client) -> remove the "or"
- consider adding an iSCSI target (server) article, or combine them in one big iSCSI article for better structure and examples.
- the network config example
- shouldn't be `/etc/netplan/50-cloud-init.yaml`, rather `.../20-iscsi.yaml`
- shouldn't require/mention disabling networking in cloud-init in `/etc/cloud/cloud.cfg.d/99-disable-network-config.cfg`
- unclear why `route-metric: 300` is adjusted (probably just remove it)
- adjust old distro standards "In Ubuntu 20.04 LTS, the default..." to rather say "since 20.04"
- "check the Device Mapper Multipath session in this same Server Guide" reword and create link
- remove `$ ` from the code block executions
- the initiator config example doesn't explain what a node is
- the example `InitiatorName` structure should be explained and made clear it can be arbitrary
- "Before configuring the Logical Units..." with no mention or explanation what a LU is.
- replace `iscsi01_ip=$(ip...` with an explanation instead of the cryptic oneliners
- if we wanted to keep oneliners, the sed replacements should be done with json instead: e.g. `ip -j a show eth0 | jq '.[0].address'`
- "Do not use this in production without checking appropriate multipath configuration options in the Device Mapper Multipathing section." => which section? link it?
- "Creating a partition:" -> rather link to a generic block device partition article, no need to explain it here again.

Start from the iSCSI initiator article referenced by the issue and review its title, network configuration, initiator configuration, multipath guidance, and partitioning sections. Clarify the terminology and examples, correct the Ubuntu and cloud-init guidance, and add or fix links where requested. Done means the article explains the general concepts and simple examples without the listed ambiguities.

Written by the indexing model from the issue text.