Add the engine build definition language.

[godofredoc]

Adds the go link for the build definition language.

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

Hi Godofredo! Did you create this doc under your @google account? I'm unable to see it on an incognito window unless I log in. For that reason, we request that people create these docs on a non @google email address.

[godofredoc]

Hi Godofredo! Did you create this doc under your @google account? I'm unable to see it on an incognito window unless I log in. For that reason, we request that people create these docs on a non @google email address.

Hi Shams,

Just shared the doc publicly, can you please give it a second try? It was created with my personal account but I wanted to read it one more time before sharing with everyone.

Thanks,
Godofredo

[sfshaza2]

lgtm!