[WIP][DNM] Add s2idle docs

[DhruvaG2000]

Document all about s2idle and PSCI and how the whole stack helps us in selecting between low power modes on the TI AM62L

[github-actions]

New warnings found with rstcheck:

source/linux/Foundational_Components/Power_Management/pm_psci_s2idle.rst:25: (ERROR/3) Unexpected indentation.

[cshilwant]

The section identifier line should be of equal length to the section name.

[DhruvaG2000]

@cshilwant , and rest I'll mark the PR ready for review once it has undergone LPM team internal review. Till then requesting you to hold off reviews.
This is going to be a major documentation update for AM62L PM story hence might take some time to get reviews and complete.

Thanks!

[github-actions]

New warnings found with rstcheck:

source/linux/Foundational_Components/Power_Management/pm_psci_s2idle.rst:48: (WARNING/2) Bullet list ends without a blank line; unexpected unindent.
source/linux/Foundational_Components/Power_Management/pm_psci_s2idle.rst:29: (ERROR/3) Error parsing content block for the "list-table" directive: two-level bullet list expected, but row 4 does not contain a second-level bullet list.

[github-actions]

New warnings found with rstcheck:

source/linux/Foundational_Components/Power_Management/pm_psci_s2idle.rst:302: (WARNING/2) Bullet list ends without a blank line; unexpected unindent.
source/linux/Foundational_Components/Power_Management/pm_psci_s2idle.rst:283: (ERROR/3) Error parsing content block for the "list-table" directive: two-level bullet list expected, but row 4 does not contain a second-level bullet list.