7245: Commit generated docs to source control r=hsbt a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that generating docs on the fly every time they are needed (tests, release, installation) is complicated and prone to errors. For example, currently the `rubygems-update` gem that takes care of updating rubygems and install the default bundler does not properly include bundler docs. Also, managing the manifest as a mix of generated and source controlled files is complicated too. Finally, ruby-core already keeps man pages as part of its sources, so this would make us follow their lead.

### What is your fix for the problem, implemented in this PR?

My fix is to keep generated man pages source controlled, and add a task to make sure they are always kept in sync with their "source of truth".

### Why did you choose this fix out of the possible options?

I chose this fix because I think it should simplify things for us.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

So that the examples work in currently supported rubies.

This has the benefit that:

* Allows the installation of bundler as a default gem from rubygems to
include man pages.
* Removes the need to build man pages during our tests.
* Makes working with the manifest easier, because we only have source
controlled files, and not a mix of source control and generated files.

To make sure they never fall out of sync, we replace the previous
`man:build` CI task with a `man:check` task that makes sure the
generated man pages are up to date.

Since we no longer use `git` to find out the list of files, the comment
is misleading.

The build task does not generate any files at `lib/bundler/generated`.