Fix serverless-dynamodb examples

[domdomegg]

The examples involving currently serverless-dynamodb-local are quite out of date. They use an older configuration API which makes it hard for new users to get started. In addition, the package serverless-dynamodb-local no longer works as the hardcoded URL it contains has broken, and it uses an old access key format that isn't compatible with new versions of DynamoDB Local. Lastly, several of the docs are just wrong in places - e.g. I suspect a copy/paste mistake has resulted in people listing the wrong endpoints for a REST API (API Gateway v1) by copying them from the HTTP API (API Gateway v2) example.

This PR:

• corrects the docs to show the correct endpoints
• updates the usage of the serverless-dynamodb package
• adds .dynamodb to gitignores to avoid accidentally committing this, given this is created by the serverless-dynamodb package
• updates the access keys in one location given AWS DynamoDB Local no longer supports underscores in fake access keys (see https://repost.aws/articles/ARc4hEkF9CRgOrw8kSMe6CwQ/troubleshooting-the-access-key-id-or-security-token-is-invalid-error-after-upgrading-dynamodb-local-to-version-2-0-or-greater)
• updates the examples to use working packages

[domdomegg]

Fixes #773

[domdomegg]

Requesting review from @medikoo

[medikoo]

@domdomegg thank for pinging. Still I'm no longer with Serverless Inc, and I don't have necessary rights to review and merge any work here.
I believe it's @ac360 who's handling the contributions now

[domdomegg]

Ah thanks! @ac360 are you able to review?