Add Greek translation for README.

[adacapo21]

Relates to
This pull request relates to issue elizaOS#2197.

Risks
Low: This PR only adds a Greek translation for the README file and updates the main README to include a link to the new translation. No functionality or core code is affected.

Background
What does this PR do?
This PR adds a new README_GR.md file containing a Greek translation of the README. It also updates the main README.md file to include a link to the Greek version under the translations section.

What kind of change is this?

• Documentation: This is a documentation change, adding a new translation and improving accessibility for Greek-speaking users.
  Documentation changes needed?
  My changes require a change to the project documentation.
  I have updated the main README file to include a link to the Greek translation.

Testing
Where should a reviewer start?
The reviewer should start by checking:

• The README_GR.md file for completeness and formatting.
• The updated translations section in README.md.

Detailed testing steps

1. Open the README_GR.md file to verify that the translation is accurate and formatted correctly.
2. Check the link to the Greek translation in the README.md file to ensure it points to the correct file.