Docs: Fix the broken links in the documentation

Sphinx uses "autosectionlable" extension for generating the section
labels automatically. The max depth is set to 2 for level 2 headers
only. That section which is under level 2 will not have the reference
label. This caused the broken links in the documentation.

This patch creates lables for the section which is under level 2 and
referenced by other places in the documentation.

Signed-off-by: David Wang <david.wang@arm.com>
Change-Id: I636922c624108e850635e683719f5ac035e0880b
diff --git a/docs/technical_references/instructions/tfm_build_instruction.rst b/docs/technical_references/instructions/tfm_build_instruction.rst
index 485d917..e2dc85d 100644
--- a/docs/technical_references/instructions/tfm_build_instruction.rst
+++ b/docs/technical_references/instructions/tfm_build_instruction.rst
@@ -12,6 +12,8 @@
 
 Cmake version ``3.15.0`` or higher is required.
 
+.. _Getting the source-code:
+
 Getting the source-code
 =======================
 .. code-block:: bash
@@ -190,6 +192,8 @@
 Note that these map directly to the ``SUITE`` cmake variable used in the
 psa-arch-tests documentation.
 
+.. _Migration from legacy buildsystem:
+
 Migration from legacy buildsystem
 ---------------------------------