Do small adjustments for freeze periods and release links API guides

What does this MR do and why?

This merge request does some small adjustments to the following API guides:

• Freeze Periods API
• Release Links API

As a result of me going over them while working on !102401 (merged) and !102406 (merged), respectively.

Changes

Below is a quick recap of the changes in the merge request:

• For Freeze Periods API guide:

  • Fix the type for freeze_period_id field.
  • Use freeze period instead of Freeze Period in section headers and description.

• For Release Links API guide:

  • Update section headers to match other similar API guides.
  • Use release instead of Release in section description.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.