This PR adds two new make targets, make dist and make doc-dist.

make dist builds an old-school source tarball from the M2 and M2-emacs git repos, and also includes a few generated autotools files so users can immediately run configure after unpacking without having to autoconf installed, similar to the make dist target in automake projects. This should be useful for creating deb and rpm packages. (I've been doing something similar on the Debian side for years.)

We also provide a backup for dealing with submodules when building from an unpacked tarball rather than a git repo. This will simplify building rpm packages, where we currently have to trick the build system into thinking that there are submodules.

Also, the make doc-dist target creates a tarball containing all of the built documentation (html, info, and "dumps" of the gdbm databases). Then, if someone runs ./configure --enable-documentation=download, the build system will download this tarball from the webpage, unpack it, and use the prebuilt documentation rather than running all the examples from all the packages. This will cut the M2 build time from hours down to minutes. This should simplify the package installation process for users using "build from source" package managers (spack, gentoo, nix, etc.). See #3905.

There is also a GitHub workflow to generate both tarballs.