Fix all the doxygen warnings.

Fix all the doxygen warnings.

• Removed non-existent examples
• Modified tags for unfound references
• Updated the Doxyfile.in and eigendoxy_layout.xml.in for latest doxygen
• Fixed various typos and punctuation mistakes
• Un-masked referenced symbols previosly guarded by #ifndef EIGEN_PARSED_BY_DOXYGEN that break because of the guard
• Removed some eigen_asserts that conflicted with constexpr function definitions
• Fixed some ODR issues in packetmath files
• Other minor changes
• Removed eigen_navtree_hacks.js, since it is outdated and no longer works at all with latest output.