fix: align setup of resource and data source id for testing framework

[lechnerc77]

Purpose

This PR contains the changes to have an aligned setup of the artificial ID field in resources and data sources that is needed for the Terraform plugin test framework. The changes comprise:

• Mark the ID field as "computed"
• Align the "DeprecationMessage" field
• Align the "MarkdownDescription"
• Use stringplanmodifier.UseStateForUnknown() in schema
• Regenerate documentation
• re-record test fixtures for resources

Closes #292

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code

go test ./...

What to Check

Verify that the following are valid

• All tests are executed successfully

Other Information

Further information is available here: hashicorp/terraform-plugin-testing#84