Docs: Add sphinx-needs migration guide and constraints

Docs: Add sphinx-needs migration guide and constraints

Add migration guide for replacing the abandoned sphinxcontrib-needs
package with its maintained successor sphinx-needs. The old package
fails on modern Python environments due to:

- pkg_resources import (removed from setuptools>=82)
- sphinx.util.status_iterator import (removed from Sphinx 8.2+)

Both issues break Read the Docs builds, which upgrade setuptools and
Sphinx to latest versions before processing requirements-docs.txt.

Changes:
- New guide: needs-sphinxcontrib-to-sphinx-needs.rst with full
  migration steps, background, and affected repository tracking
- upper-constraints.onap.txt: Add sphinx-needs==7.0.0 pin
- diagrams-blockdiag-to-mermaid.rst: Add seealso cross-reference
  to the new sphinx-needs migration guide
- index.rst: Add new guide to documentation toctree

Issue-ID: CIMAN-33
Change-Id: Iaddb82bd98a92d394f480059643b4f7cf87e4d64
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>