Add documentation for overriding QM attributes using drop-in files

[ArtiomDivak]

"Add documentation for adjusting OOM scores in nested containers

This adds comprehensive documentation on how to customize OOM score
adjustment for nested containers running inside QM, including:

• Step-by-step configuration of nested container Quadlet files
• QM drop-in configuration for required capabilities
• Verification process using podman-system-generator
• Testing procedures with expected OOM score values

The documentation shows how to set different OOM priorities for
nested containers (1000 vs 100) to control OOM killer behavior.

Closes #899

[sourcery-ai]

Reviewer's Guide

Adds a new documentation section in README.md detailing how to override QM container attributes via systemd drop-in files, complete with setup steps, an example use case, verification procedures, and best-practice notes.

File-Level Changes

Change	Details	Files
Introduce 'Overwriting QM attributes' documentation section	Define drop-in directory path and file naming convention Outline steps to create directory and custom .conf file Explain persistence across QM updates	README.md
Provide example for removing device access	Show drop-in file content to clear AddDevice entries Contrast default AddDevice settings with overrides	README.md
Add verification procedure for drop-in overrides	Include dry-run command for podman-system-generator Advise checking generated ExecStart output Explain service reload steps	README.md
List important notes on drop-in file ordering and naming	Emphasize alphabetical processing of drop-in files Recommend descriptive filenames for order control Advise verifying changes before applying	README.md

Possibly linked issues

• Document how to change OOMScoreAdjust for nested containers #899: The PR adds comprehensive documentation on customizing QM container configuration using drop-in files, directly addressing the issue's request.

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[sourcery-ai]

Hey @ArtiomDivak - I've reviewed your changes and they look great!

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[dougsland]

We are looking to document OOMScoreAdjust for nested containers from #893, please update your PR to document 893.

[Yarboa]

/packit test --identifier kvm-tier-0

[Yarboa]

@ArtiomDivak please take a look at aib test
https://artifacts.dev.testing-farm.io/0dbe8127-fd62-47e1-a585-613fcbcd9053/

Can you analyze it?

There is another fix need to be rebased once it is merge

[Yarboa]

@ArtiomDivak please take a look at aib test https://artifacts.dev.testing-farm.io/0dbe8127-fd62-47e1-a585-613fcbcd9053/

Can you analyze it?

There is another fix need to be rebased once it is merge

looking at it on #916