* A first attempt at overhauling the documentation job with a cache.
* Actually remove files we don't need anymore.
* Fix output from local link checks to give it all at once.
* Add sparse matrices to the set of detected matrix types.
* Fix comparison condition.
* Make a pass to try and re-validate cached links that have not yet expired.
* Compile and link separately so that ccache can take effect.
* Be a little bit smarter about caching.
* Ensure libicu$SO is installed so 'stringi' can be used
Also collapse to remotes calls into one
* Fix style issues that cpplint 2 found (#3884)
* Try to workaround the libicu issue for now.
* Mention new dependency.
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Co-authored-by: Dirk Eddelbuettel <edd@debian.org>
