Clarify limitations linked to standalone orgs VS GitHub App pipelines

[BeFunes]

Description

I changed some of the content of the "Currently not supported" section of the GitHub App integration page (https://circleci.com/docs/guides/integration/github-apps-integration/#currently-not-supported).

Reasons

Often our docs conflate the limitations linked to "GitHub App pipelines" with those linked to standalone organizations.
With an ever growing number of (paying) GitHub OAuth organizations adoption GitHub App pipelines, it's important to clarify which of these limitations apply to them and which don't.
As a matter of fact, most of the limitations listed in the GitHub App integration page actually are not truly tied to the GitHub App integration, but rather to the organization type (= /circleci/ instead of /github/).

I made these changes directly on the GitHub App integration page as a first urgent step, but I think we should consider organizing this content better and perhaps having a dedicated Org type disambiguation page.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Include relevant backlinks to other CircleCI docs/pages.