Clarify `tmt` mission and compare with others

[lukaszachy]

Basically follow up to #1218 (reply in thread)

I think we feel what tmt should be about but it is hard to put it down.
So we know what is in the scope, what is just a "good will" to make life easier, who do we "compete" with and why we win/lose.

IMHO Miro's "imagine running travis or gitlab-ci yaml from your localhost" captures most of it, but there are bits tmt is better or worse with and we should have them written down.

[psss]

I'd say that the description section quite nicely summarizes the main tmt goal and features:

• https://tmt.readthedocs.io/en/stable/overview.html#description

@lukaszachy, do you think it would be still worth adding some comparison with other tools to clarify the scope even more?

[psss]

From triage meeting: This is something to clarify as part of defining the tmt vision.