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new command index *only* if FILE is out of date w.r.t. commands in the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.
6380f1917a