Add initial docs for the engine build definition.

[godofredoc]

This is an initial convertion to markdown from flutter.dev/go/engine-build-definition-language.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[chinmaygarde]

LGTM. But this could also perhaps be in the wiki.

[godofredoc]

LGTM. But this could also perhaps be in the wiki.

Thanks, I'll link this doc to the wiki. The main idea is to keep the docs closer to where they are being used to ensure they are getting updated when something on the definition language implementation changes.