docs: define vendor-data and meta-data in QEMU tutorial

[nightcityblade]

Proposed Commit Message

docs: Define QEMU vendor-data and meta-data

Explain how provider defaults interact with user-data and how changes to
the instance ID affect first-boot detection.

Fixes GH-4947

Additional Context

The QEMU tutorial already names the three configuration data sources, but it
does not explain the operational role of vendor-data or why the instance ID in
meta-data matters. This adds those details where the data files are introduced.

Test Steps

• PATH="$PWD/.tox/doc/bin:$PATH" READTHEDOCS=True .tox/doc/bin/python -m sphinx -W --keep-going -n --jobs=auto doc/rtd doc/rtd_html
• .tox/doc/bin/python -m doc8 doc/rtd
• git diff --check

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

This looks good to me.

[holmanb]

@nightcityblade please sign the CLA.