Add information about getting started to the overview section.

[Shy]

Hey folks, I'm working at an org that is rolling out support for our users to upload OpenAPI specs to our product when working with rest integrations. We've been testing while utilizing y'alls spec and it works great. I would love for my users to get info about how actually get an API key when they import your spec though. I've added the following to the top of the YAML file I'm importing while I'm testing things and figured it might be an easy thing for y'all to add:

openapi: 3.0.3
info:
  version: 1.1.4
  title: GitHub v3 REST API
  description: |-
    GitHub, the leading platform for version control and collaboration, revolutionizes software development. Integrate GitHub to elevate your development process and collaboration capabilities. Effortlessly manage repositories, streamline code reviews, and manage releases to accelerate your team's productivity.

    ## Get started
    To get started you'll need a GitHub access token. GitHub has many authentication options. The fastest way to get started is using a Personal Access Token (PAT). To create a token:

    1. Log in to GitHub
    2. Navigate to **Settings** → **Developer Settings** → [**Personal access tokens**](https://github.com/settings/tokens?type=beta)
    3. Click **Generate new token**
    4. Name your token and configure [access & permissions](https://docs.github.com/en/enterprise-cloud@latest/rest/overview/permissions-required-for-fine-grained-personal-access-tokens)
    5. Click **Generate token**
    6. Copy the API key to configure your integration's connection

    Learn more about [GitHub API Authentication](https://docs.github.com/en/enterprise-cloud@latest/rest/overview/authenticating-to-the-rest-api)