Eigen website down

[oskbjo]

Not an issue with ORB-SLAM3, but an issue with a prerequisite.
The website hosting the download and install instructions for Eigen3, http://eigen.tuxfamily.org/, is down, at least currently. The code is available at https://gitlab.com/libeigen/eigen/-/tree/3.4?ref_type=heads, and the install instruction are available in the INSTALL file. Might be worth updating the Prerequisites with the gitlab site instead if the Eigen website continues being unreachable.

[qleonardolp]

Eigen3 website is officially transitioning to a GitLab pages: https://libeigen.gitlab.io/
Docs: https://libeigen.gitlab.io/eigen/docs-3.4/