Convert the developer guide to docusaurus.

[dckesler]

The relevant documentation in the gitbook format is inside of docs/developer-resources. The urls should match the current documentation starting with https://docs.celo.org/developer-guide/overview.

[dckesler]

• The relevant files are found in docs/deverloper-resources/
• We should match the urls starting at https://docs.celo.org/developer-guide/overview
• Later the SDK References are going to be completely pulled out into a different landing page but we'll eventually want a sidebar link that goes there when it's ready. In the meantime we can probably just leave out the SDK References part at https://docs.celo.org/developer-guide/sdk-code-reference