ESC: Break out the Syntax Reference into individual pages

[jkodroff]

The Syntax Reference was created early in ESC's lifecycle. Now that ESC is a top-level member of the platform, we should document ESC environment syntax in a more refined way that is easier to read.

Some notes:

• Integrations/providers examples are already covered elsewhere, but we should ensure each one has complete, functioning environments, not just snippets. See: https://www.pulumi.com/docs/esc/integrations/dynamic-login-credentials/aws-login/#example
• Interpolation is great! It should get its own section because it's quite useful! Call it out!
• Each export type should have its own full (but simple) example.

Maybe consider combining the basic syntax in the same section as the integration/provider reference, e.g. "Environments Syntax Reference" or something like that.