Fix PostgreSQL resource name display in documentation sidebar

[Copilot]

Fixes the inconsistent resource name display in the Terraform provider documentation sidebar. The sidebar was showing gridscale_postgres while the actual resource name is gridscale_postgresql.

Problem

The Terraform registry documentation displayed an incorrect resource name in the left sidebar navigation. Users would see gridscale_postgres in the sidebar index, but the documentation content and all code examples correctly used gridscale_postgresql.

Root Cause

The issue was in website/gridscale.erb where the sidebar link text incorrectly displayed gridscale_postgres instead of the correct gridscale_postgresql resource name.

Solution

Updated the sidebar display text in website/gridscale.erb to show the correct resource name gridscale_postgresql, ensuring consistency between the navigation and documentation content.

This change affects only the documentation display and does not modify any functional code or resource implementations.

Resolves #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Docs: wrong name of PostgreSQL resource page</issue_title>
<issue_description>The docs available on Terraform's registry list the resource name as gridscale_postgres in its index on the left, even though the contents on the right use the correct name gridscale_postgresql. My guess is that this is due to the corresponding file having the wrong name.</issue_description>

Comments on the Issue (you are @copilot in this section)

@nvthongswansea Thank you @mbunkus for reporting this issue.

Fixes #194

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