[a11y] Descriptive Alt Test

[shiffman]

I'm noticing that alt text for some images appears to be placeholder content and isn't descriptive, for example, on the getting started page:

<center>
    <img alt="pose detection" src="assets/gettingstarted-sketch-folder.png">
</center>

Create a new folder called `images`.

<center>
    <img alt="pose detection" src="assets/gettingstarted-create_folder.png">
</center>

And upload a bird image named `bird.png` to the `images` folder. Remember to login to see this option.

<center>
    <img alt="pose detection" src="assets/gettingstarted-upload-file.png">
</center>

[QuinnHe]

getting started page updated, other pages will be updated as we modify the content