chore: set tf docs plugin version to 0.18.0

[lechnerc77]

Purpose

• Workaround for tf-docs-plugin issue as described in https://github.com/hashicorp/terraform-plugin-docs/issues/355

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)
[ ] Documentation content changes
[X] Other... Please describe: version pinning

How to Test

make generate

What to Check

Verify that the following are valid:

• Automated tests are executed successfully: header of docs contains links and bullet lists (see e.g. resource for directory

Other Information

• See GitHub Action run for diff: https://github.com/SAP/terraform-provider-btp/actions/runs/8698319409/job/23855116473
• Follow-up issue is #774

Checklist for reviewer

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

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