docs/sphinx: Fix FUSA indexing

docs/sphinx: Fix FUSA indexing

Sphinx complains:

  docs/fusa/index.rst:6: WARNING: toctree contains reference to nonexisting document 'fusa/reqs'
  docs/fusa/reqs/index.rst:6: WARNING: toctree contains reference to nonexisting document 'fusa/reqs/market-reqs'
  docs/fusa/reqs/index.rst:6: WARNING: toctree contains reference to nonexisting document 'fusa/reqs/product-reqs'
  docs/fusa/reqs/index.rst:6: WARNING: toctree contains reference to nonexisting document 'fusa/reqs/design-reqs/arm64'

  docs/fusa/index.rst: WARNING: document isn't included in any toctree
  docs/fusa/reqs/design-reqs/arm64/generic-timer.rst: WARNING: document isn't included in any toctree
  docs/fusa/reqs/design-reqs/arm64/sbsa-uart.rst: WARNING: document isn't included in any toctree
  docs/fusa/reqs/index.rst: WARNING: document isn't included in any toctree
  docs/fusa/reqs/market-reqs/reqs.rst: WARNING: document isn't included in any toctree
  docs/fusa/reqs/product-reqs/arm64/reqs.rst: WARNING: document isn't included in any toctree

Fix the toctrees.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>