Improve main README.md

[promisinganuj]

This issue consolidates the feedback from the team around the documentation for this sample, especially the main README.md.

DoD

Update main README.md to:

• Explain that for SP, FABRIC_CAPACITY_ADMINS requires the "Object ID" of the enterprise application associated with the app, not the "Object ID" of the app itself.

• Fix the image alignment

• Make it clear that the "devcontainer" can be used for the deployment, and also that it's the recommended way. Perhaps, add instruction on how to use "devcontainer" like it's explained in Databricks parking sensors sample.

• Improve the prerequisites section:

Once that was mentioned in the pre requisites it would not need to be mentioned in the authentication, it could just be referred with a link to pre- requisites.

• Add details about provider registrations for the subscriptions.

│ Error: MissingSubscriptionRegistration: The subscription is not registered to use namespace 'Microsoft.OperationalInsights'. See [https://aka.ms/rps-not-found](https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Faka.ms%2Frps-not-found&data=05%7C02%7Canujparashar%40microsoft.com%7C29f53c076ee742e0e39c08dd0e340fc9%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C638682339021888003%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=FoKwH5UBGxHczDaH8UaWm7P8ygjSfxo5Lth%2FLUiKlgI%3D&reserved=0) for how to register subscriptions.”

• Add more details to the variables, like a practical example and a screenshot:

```GIT_REPOSITORY_NAME, GIT_BRANCH_NAME, GIT_DIRECTORY_NAME ...`