Document process for "Preview" API endpoint usage

[bitwiseman]

We need to document how the project supports using "Preview" API endpoints, including @Preview, @Deprecated, withPreview(), and the Previews class.

Also all @Preview items should include a link of GitHub documentation of what they are previewing and @deprectated JavaDoc field with boiler plate explaining that this feature is still in preview and a link to the same documentation.

[bitwiseman]

These are preview api:
https://docs.github.com/en/free-pro-team@latest/rest/overview/api-previews

API previews let you try out new APIs and changes to existing API methods before they become part of the official GitHub API.

During the preview period, we may change some features based on developer feedback. If we do make changes, we'll announce them on the developer blog without advance notice.

To access an API preview, you'll need to provide a custom media type in the Accept header for your requests. Feature documentation for each preview specifies which custom media type to provide.

To see examples of how previews work in this library, look at any method that uses the withPreview() method such as:

github-api/src/main/java/org/kohsuke/github/GHApp.java

Lines 200 to 220 in a585b49

	/**
	* Obtain an installation associated with this app.
	* <p>
	* You must use a JWT to access this endpoint.
	*
	* @param id
	* Installation Id
	* @return a GHAppInstallation
	* @throws IOException
	* on error
	* @see <a href="https://developer.github.com/v3/apps/#get-an-installation">Get an installation</a>
	*/
	@Preview(MACHINE_MAN)
	@Deprecated
	public GHAppInstallation getInstallationById(long id) throws IOException {
	return root.createRequest()
	.withPreview(MACHINE_MAN)
	.withUrlPath(String.format("/app/installations/%d", id))
	.fetch(GHAppInstallation.class)
	.wrapUp(root);
	}

[bitwiseman]

Related #1003

[bitwiseman]

Almost all previews have be graduated. There is a new system for API versioning in GithHub. We will need to do something similar to this issue for the new scheme.