Document how to sign mac builds (StandaloneOSX)

**Context**

Mac builds require `codesign` in order for users to be able to be easily download and run.
Users are confused about why they are unable to download and run their mac builds.

**Suggested solution**

Create a page at https://game.ci/docs/github/deployment/mac that would detail the `codesign` process, and how to upload builds to the Mac App Store.

**Considered alternatives**

Per https://github.com/game-ci/unity-builder/issues/329, users may be able to bypass code-signing by running these commands on the downloaded artifact:
```
chmod +x ~/Downloads/StandaloneOSX.app/Contents/MacOS/
xattr -d com.apple.quarantine ~/Downloads/StandaloneOSX.app/Contents/MacOS/
```

But it would be better to teach users how to sign their app, so that they are able to distribute their app.

**Additional details**

I share my `codesign` process here: https://github.com/game-ci/unity-builder/issues/329#issuecomment-1079553006