doc: tfm: Add documentation regarding configurable build

[SeppoTakalo]

Add documentation regarding CONFIG_TFM_PROFILE_TYPE_NOT_SET and various TF-M partitions that user need to configure.

NOTE: This is very early phase. This work is still in progress, but I'm just starting the review process very early.

[NordicBuilder]

CI Information

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Build number: 61

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Enabled	Name	Description
	ci-disabled	Disable the ci execution
	ci-all-test	Run all of ci, no test spec filtering will be done
	ci-force-downstream	Force execution of downstream even if twister fails
	ci-run-twister	Force run twister
	ci-run-zephyr-twister	Force run zephyr twister

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Test Spec & Results: ✅ Success; ❌ Failure; 🟠 Queued; 🟡 Progress; ◻️ Skipped; ⚠️ Quarantine

• ◻️ Toolchain
• ◻️ Build twister
• ◻️ Integration tests

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[NordicBuilder]

You can find the documentation preview for this PR at this link. It will be updated about 10 minutes after the documentation build succeeds.

Note: This comment is automatically posted by the Documentation Publish GitHub Action.

[MarkusLassila]

Some comments. Moving to right direction.

[peknis]

More comments/suggestions coming later this week.

[greg-fer]

Almost there :)

[greg-fer]

Looks good. As discussed with both Seppo and Markus, more improvements can be made, but we can tackle them in separate PRs.