Make use of `git --man-path` and `git --html-path` to simplify the
manual installation instructions a bit. Also, there appears to be a
site.getsitepackages() call in python to give similar information about
where git_filter_repo.py can be installed.
Signed-off-by: Elijah Newren <newren@gmail.com>
Now that several package managers are packaging filter-repo (Debian and
Ubuntu seem to be the primary holdouts, but maybe treating Linux as
"covered" will pressure them to package it too), guide people to use
package managers for easy installation and streamline the wording.
Still keep the old instructions around, just move them later.
Signed-off-by: Elijah Newren <newren@gmail.com>