Merge pull request #1818 from pbeesley-arm/doc-links

doc: Fix broken external links

diff --cc docs/change-log.rst
Simple merge

diff --cc docs/cpu-specific-build-macros.rst
Simple merge

diff --cc docs/platform-compatibility-policy.rst
Simple merge

diff --cc docs/porting-guide.rst
Simple merge

diff --cc docs/trusted-board-boot.rst
Simple merge

diff --cc docs/user-guide.rst
Simple merge