docs: fix wrong link for API credentials

[lechnerc77]

Purpose

• Add link from help.sap.com for API credentials resource
• Closes [BUG] Incorrect Documentation Link for "Further Documentation" in btp_subaccount_api_credential Resource #912

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

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

How to Test

• Test the code via automated test

go test ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

• Check markdown to contain same link as help.sap.com

Other Information

Validated layout of markdown files at https://registry.terraform.io/tools/doc-preview

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[diya-dhan]

Changes look fine to me