rel=canonical  is supposed to be helpful for search engine to knowwhich pages a search engine should promote.This is _not_ a redirect.If a file exists and is and is not in a versioned (x.y.z) folder; consider itas a potential "canonical" targetIf a file exist in a versioned doc of matplotlib make its rel=canonicalpoint to the corresponding candidate.Example:    path/to/foo.html -> candidate as being canonical    3.0.1/path/to/foo.html > set rel=canonical to matplotlib.org/path/to/foo.htmlSpecial case the `examples` and `gallery` folder.The `example` folder is from old matplotlib version (2.0.2) but stillquite popular on Google; special case.1) Apply the same rule as above but from withing `examples` toward`gallery`:    /gallery/path/to/foo.html -> candidate as being canonical    /example/path/to/foo.html > set rel=canonical to matplotlib.org/gallery/path/to/foo.html    This took care of 145 files.2) Many examples were "moved" during examples -> gallery transition. Iffilename is unique in gallery; use this as a potential heuristic fordetecting move:    /gallery/path/to/verryuniquename.html -> candidate as being canonical as the filename is unique !    /example/old/location/verryuniquename.html > set rel=canonical to matplotlib.org/gallery/path/to/verryuniquename.html    This took care of 185 files in example    Leaving 271 "orphan"3) Resolve chain of rel-canonical. Fix links that point to examplethat themselves point to gallery.    /gallery/path/to/verryuniquename.html    /example/old/location/verryuniquename.html ->  matplotlib.org/gallery/path/to/verryuniquename.html    /2.0.2/example/old/location/verryuniquename.html -> should point to /example/....                                                       make it point to /gallery/....No files with existing rel=canonical have been touched. This willlikely only affect pre-3.0 documentation files.