(docs) VM.migrate.md: Rephrase and simplify, improve readability

[bernhardkaindl]

(docs) Update the walk-trough VM.migrate:

• Include the Live migration flowchart for reference and as overview.
• Fix the chapter structure and chapter headings: Improved table of contents (1st button in the top bar)
• Add chapter links to the mentioned atomics operations
• Covert a long sentence describing a list of parameters into a simple list of parameters
• Clarify ambiguities, e.g. "if we are already at the right place" -> "if the command is already at the destination host"
• Removed the use of the word "will" when things are already happening or are done just at that point.
• Removed sentences that just filled space by just mentioning what the next chapter headline says.
• Improved the chapter on the final step with more references and a link to a helpful explanation.
• Updated the links for a better understanding to the currently used URLs.

[bernhardkaindl]

Here is a preview link to the revamped walk-though for VM_migrate:
https://bernhard-xapi-onrender-com-pr-15.onrender.com/xenopsd/walkthroughs/VM.migrate/

JFYI, as you commented on my workflow on VM.build: cc @stormi (also @gthvn1 and @ydirson)!

This PR just presents everything in an updated way, there is no actual change to the described content, except for more references and headings.

Just to clarify, this is not a new workflow, but with the updates to the chapter headings, conversion of text paragraphs into lists and the cleanups and with including the sequence diagram, it looks like a big update :-)

PS: Here is the preview of the updated table of contents that the ToC button (left in the top bar) shows:

• Invocation
• Overview
• Preparing VM migration
• Importing metadata
• Memory setup
• Destination VM setup
• Destination VM restore
• Destination VM rename
• Restoring devices
• Creating the device model
  • PCI plug
  • Unpause
• Cleanup
• Live migration flowchart
• References

[gthvn1]

cool improvements 👍