docs: (tasklist) update tasklist-rest-api overview

[ce-dmelnych]

Description

This PR addresses the issues raised in ticket #2310.

The changes made include the correction of incorrect HTTP methods, with GET replacing POST where necessary.
The Save draft task variables endpoint has been added for the endpoints overview for the next version.
In addition, for both 8.2 and next versions, a link has been added to each endpoint, redirecting users to a more detailed description of the endpoint. This should assist with providing more comprehensive information about each endpoint.

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[ce-dmelnych]

Fix for 8.2 version:

Fix for the next version: