Update documentation for 1.4.6

Add some warnings about the fact Par4All is no longer supported by SILKAN.
Update the download section to use up-to-date GitHub instead of SILKAN
servers.
Update the changelog and version number.


Former-commit-id: 20ca06a1f2ecc71598658bb43de6b8cbc27b5d69
Extend publication list on Par4All web page


Former-commit-id: 8f1d3db33b850630265eeb899b55f6b89e322004
Publish new Par4All WWW site version


Former-commit-id: 521c63720b3c13663c0d0c193bdf72a9ea9e8234