Add instructions to run a Github App pipeline via API

[BeFunes]

Description

I've edited the section Trigger a pipeline using the API to:

• Updated the TIP section about lack of support for BBDC, Github App & Gitlab to only mention Gitlab
• Added a section about how to trigger a Github App / BBDC pipeline via API, including a CURL command

I've not been able to preview these changes

Reasons

We are introducing support for triggering pipelines via API for Github App and BBDC pipelines.
This is supported by a new endpoint, different from the one used by classic projects.
In ~1 week, this new endpoint will also be usable for running classic pipelines. Once that's shipped, documentation will go live on https://circleci.com/docs/api/v2/index.html.
@BeFunes will sync with @rosieyohannan to update documentation once this goes live.

Content Checklist

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[rosieyohannan]

I think it would be better to explain project-slug here rather than leaving people to find it on the other page, is that ok if I add that?

[BeFunes]

Of course! Thank you