Replace default team with world team

[minsii]

This PR contains two changes:

• Replace default team with world team. Resolves Replace default team with world team openshmem-org/specification#371 for the teams and context sections.
• Move the introduction part into a separate file, similar to other sections.

[minsii]

@jdinan @agrippa @naveen-rn @wrrobin could you please review?

[jdinan]

@minsii How do you want to handle updating default -> world team for the other sections?

[minsii]

@jdinan I have submitted separate PRs to the other section committee branches. Some of them have been already merged. You can see the PR list at openshmem-org#371

[jdinan]

Thanks, I had missed those other PRs. Did you get the instance of "default team" in the constants section? This is the place where the term is associated to SHMEM_TEAM_WORLD.

[minsii]

Did you get the instance of "default team" in the constants section? This is the place where the term is associated to SHMEM_TEAM_WORLD.

Yes, it has been added into the front matter section.