Splitting webhook integration content over separate pages

[yaningo]

Description

Splitting webhook integration content over two separate pages:

• Custom webhooks (under "Orchestrate and trigger > Trigger")
• Outbound webhooks (under "Integration > Integration features")

Reasons

Having both types of webhooks on the same page was confusing.

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]

Big improvement, thank you!

[rosieyohannan]

I think this line needs to be in the section for examples below the quickstart?