Refactor kernel command-line options documentation

[farshidtz]

The following add PR overlapping information:

• 13

• 14

Some content should be moved from the how-to to reference(s).

Other needed fixes:

• Reference the internal how to instead of external link: https://github.com/canonical/real-time-ubuntu-docs/blob/bb848f9c12a218317069a5a757de33b7e88d9044/docs/how-to/tune-irq-affinity.rst?plain=1#L131
• CPU config how to suggests appending params to GRUB_CMDLINE_LINUX_DEFAULT while IRQ affinity how to suggests GRUB_CMDLINE_LINUX