Add ability to find broken links

[sipke]

Add tools to check for dead links in the documentation, if such tools do not already exist and this is feasible.

[sipke]

Links created by sphinx :ref: are already listed as warnings when the label is not found.
WARNING: undefined label: 'boards'
For external links, sphinx has a builder checklink which can be run to flag any broken links.
Not all broken links are actually broken so a manual verification is still needed.

PR #88 is to add this checklink builder and correct existing links in openamp-docs.

executed as
SPHINXOPTS="-b linkcheck" make html
output like
(demos/system_reference-AMD-Xilinx: line 9) redirect https://www.xilinx.com/products/boards-and-kits/ek-u1-zcu102-g.html - permanently to https://www.amd.com/en/products/adaptive-socs-and-fpgas/evaluation-boards/ek-u1-zcu102-g.html
(demos/hvl_virtio: line 18) ok https://www.zephyrproject.org/ (lopper/specification/source/references: line 34) broken https://xenproject.org/developers/teams/xen-hypervisor/ - 404 Client Error: Not Found for url: https://xenproject.org/developers/teams/xen-hypervisor/