commands: Add --header and --update options to `spack commands`

The Spack documentation currently hard-codes some functionality in
`conf.py`, which makes the doc build less "pluggable" for things like
localized doc builds.

In particular, we unconditionally generate an index of commands and a
package list as part of the docs, but those should really only be done if
things are not up to date.

This commit does the following:

- Add `--header` option to `spack commands` so that it can do the work of
  prepending text to its output.

- Add `--update FILE` option to `spack commands` that makes it generate a
  new command index *only* if FILE is out of date w.r.t. commands in the
  Spack source.

- Simplify code in `conf.py` to use these options and only update the
  command index when needed.

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