charter: Define "TDC Members" and remove non-example "maintainers"

[wking]

We never formally define "TDC Maintainers". They're just an example
role, and 4.b.viii.1 allows the TDC to define it's own roles. This
commit replaces normative references to "maintainers" and
"contributors" with TDC Members, and tries to clarify TDC Role-holders
as a subset of TDC Members.

It's unclear how TDC Roles are established and maintained (e.g. maybe
only Maintainers get to vote on adding new Roles or inducting new
Maintainers). Presumably initial the set of TDC Members can reach an
agreement on the initial rule adoption, and further rule changes can
be governed by that agreement. In the event that they cannot reach an
agreement on the initial rule set, I guess they can appeal to the TOB
(5.a), although that would have to wait until after the TDC and OCI
Members elect the TOB ;).

This PR will have syntactic conflicts (but no semantic conflicts) with
the series of PRs ending with #4. I'm happy to rebase either series
if one of them lands before the other.

See also #5, where I point out some confusion about figuring out the
TDC Member set using runC-side maintainer listings. The distinction
floated here (TDC Role-holders are a subset of TDC Members) would
allow @vbatts and @philips to be TDC Members without being
maintainers, although the charter would still have to be edited to add
@cyphar to the initial TDC-Member set (assuming the subdirectory
MAINTAINERS files list TDC Role-holders).

[wking]

OCI members can figure this out amongst themselves, and don't have a channel for input from external parties like me.