Docs: Update SW requirements documentation

- Remove any mentions of Cygwin or MinGW need for Windows build
- Restructure the document
- Add tools/requirements.txt file for installing Python packages

Change-Id: I3dfb710424f0ebd87b335e21962fa66b9fb9e244
Signed-off-by: Anton Komlev <anton.komlev@arm.com>
diff --git a/docs/getting_started/tfm_build_instruction.rst b/docs/getting_started/tfm_build_instruction.rst
index 503dd8b..70e6f86 100644
--- a/docs/getting_started/tfm_build_instruction.rst
+++ b/docs/getting_started/tfm_build_instruction.rst
@@ -26,7 +26,7 @@
 .. Note::
 
  - For building with Armclang compiler version 6.10.0, please follow the note
-   in :ref:`docs/getting_started/tfm_sw_requirement:External dependencies` section.
+   in :ref:`docs/getting_started/tfm_sw_requirement`.
  - For building with the IAR toolchain, please see the notes in
    :doc:`software requirements <tfm_build_instruction_iar>`