[template] Motivation and Background should be optional

[plehegar]

Not all charters need a motivation and background section, especially if this is a charter for an exciting group.

Relates to #355

[samuelweiler]

As historical context, on 10 February 2023, this section title was changed from "Background" to "Motivation and Background" and the word "brief" was removed. This appears to have been done as a commit, not a PR.

Some of the discussion behind this can be found in
#383.

In #383 (comment), I said "Since this was merged(sic) while I was away, I won't reopen - maybe there was a consensus (e.g. on a call) not reflected in this issue, but I will be pushing back in individual cases."

Looking more at the history, I'm thinking the change in fc3234f did, in fact, not have consensus.

[AramZS]

I concur. This should be optional. Seems like a well defined and clear mission and scope would have a lot of overlap with this section.