Configuration source and trigger docs refresh

[rosieyohannan]

Description

• Add full descriptions and setup steps for config sources and triggers to the pipelines overview
• Move conifg source and trigger info from integrations pages to the pipelines overview and replace with links
• Update screenshots

Reasons

Help customers get a full understanding on what config sources and triggers are and how to set them up

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).
• Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• Include relevant backlinks to other CircleCI docs/pages.

[rosieyohannan]

Line 19-34 already existed so no need to review at this time, changes are needed but out of scope for this PR. I've converted the file so it looks like the whole file is new.

[sebastian-lerner]

i found some typos but no major issues from my end

[sebastian-lerner]

it would be good to have benny give it a review as well and make sure she has no concerns

[BeFunes]

Looks good!