Update and rename united_kingdom.md to great_britain.md

[arzoo0511]

Description
This PR renames united_kingdom.md to great_britain.md and adds three more datasets related to Great Britain's electricity grid data. The additional datasets enhance the coverage of real-time, historical, and forecasted energy data, making the resource more comprehensive.

Fixes #13 #11

How Has This Been Tested?
The changes involve adding documentation and do not affect code functionality. However, the following checks were performed:

Verified all dataset links are accessible and working.
Ensured proper Markdown formatting for readability on GitHub.
Reviewed content to ensure consistency in structure and information.

Checklist:
My changes follow [OCF's coding style guidelines]
I have performed a self-review of my own changes.
I have made corresponding changes to the documentation.
I have checked my work for accuracy and consistency

[zakwatts]

Thanks for doing this, The formatting, seems quite off though from the original grid_templates.md file. would you be able to have this match that?

[zakwatts]

Thanks for submitting these changes. A couple of comments

[arzoo0511]

made all the necessary changes , pls view

[zakwatts]

Thanks!