Clean up datasource template

This is a follow-up issue for #5000 where we moved datasource config from JSON files to TS files and discovered few aspects of the datasource template that can be improved.

• Update https://github.com/strongloop/loopback4-example-shopping to follow the new style
• Update https://github.com/strongloop/loopback-next/blob/master/docs/site/Inside-Loopback-Application.md to follow the new style
• Make the static DataSource property name read-only, set it from config.name to avoid duplication.
• Rework datasources to receive the configuration via @config, so that they can be configured via app.configure(). Update the docs (especially Cloudant & Bluemix guides), make sure to preserve content for the old configuration style (think of existing projects created before the change).
• Fix datasource files in examples to use the same style as scaffolded by lb4 datasource. Most notably, change them to participate in app life-cycle and disconnect when the application has stopped. Update the docs accordingly.
• Verify that integration tests instantiating datasource classes directly are stopping them after the tests are over + update the relevant docs (e.g. testing best practices).
• Remove start/stop code in favor of stop() method provided directly by juggler, see https://github.com/strongloop/loopback-datasource-juggler/pull/1835 and #5279

Acceptance criteria

For each of the items above, consider the following places to update

• The template in packages/cli/generators/datasource/templates/datasource.ts.ejs
• The template used by CLI tests - see getSourceForDataSourceClassWithConfig()
• All documentation pages in docs/site, do a text search to find all snippets showing a datasource
• All example applications in examples

🎆 Hacktoberfest 2020

Greetings 👋 to all Hacktoberfest 2020 participants!

Here are few tips 👀 to make your start easier, see also #6456:

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