Remove all uses of headings lower than h4

[rosieyohannan]

Description

Changed heading levels to stop using h5 and h6 for better navigation and readability

In the configuration reference I have flattened some sections to h4, other sections I've moved up to h2, for example the execution environments section and jobs under workflows.

Reasons

After talking with design its been decided we will only use h1-h4

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.