CTRT edits for GitHub page - part 1

What does this MR do?

First round of CTRT edits for the GitHub page:

• Updated H1 title to reflect the real purpose of this page (which is to set up GitHub as an OAuth provider).
• Split the 'Enabling GitHub OAuth' section into two distinct tasks - one that must be done in GitHub, and one in GitLab. The two sections are:
  • Create an OAuth app in GitHub
  • Enable GitHub OAuth in GitLab
• Renamed and moved the 'Security check' section to be a subsection of 'Create an OAuth app in GitHub' as it relates to configuration that you do in GitHub.
• Cleaned up the steps to update the GitLab config file.

Before: https://docs.gitlab.com/ee/integration/github.html

After: http://main-ee-80073.178.62.207.141.nip.io/ee/integration/github.html

(I will do the 'GitHub Enterprise with self-signed Certificate' and 'Troubleshooting' sections in another MR.)

Related issues

technical-writing#531 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.