Doc update: post build now supported on TF-M targets

[LDong-Arm]

Summary of changes

With the merging of #14361, post build operations are now supported on all targets that require them including TF-M targets.

Impact of changes

Documentation update.

Migration actions required

None.

Documentation

Updated by this PR.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@0xc0170 @hugueskamba @rajkan01

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[LDong-Arm]

@0xc0170 This is a doc-only update, so CI shouldn't be needed I guess.

[ciarmcom]

@LDong-Arm, thank you for your changes.
@hugueskamba @rajkan01 @0xc0170 @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[0xc0170]

With the merging of #14361, post build operations are now supported on all targets that require them including TF-M targets.

Awesome, this means all target should be now ported for the upcoming release 🎉