Documenting user SSH key addition to a GiHub OAuth project

[yaningo]

Description

Adding instructions on how to add a user SSH key to a GiHub OAuth project.

Reasons

In the current version of the documentation, this step isn't currently explicitly outlined (unlike the case of Bitbucket projects).

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.
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• 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).
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