Move documentation from GitHub Wiki to repository for better maintenance

[coldic3]

Currently, our documentation (docs, installation guide, etc.) is hosted in the GitHub Wiki at https://github.com/CommerceWeavers/SyliusTpayPlugin/wiki.

Problem:

This approach creates significant maintenance challenges since there's no code review process for documentation changes, documentation updates aren't linked to code changes, and it's much harder to maintain docs in sync with code evolution. Additionally, we miss the opportunity to review documentation alongside related code changes and lack proper version control integration with our standard pull request workflow.

Proposed Solution:

Moving documentation into the main repository (such as a /docs folder) would enable documentation reviews through our standard PR process, allow atomic updates where code and docs change together, improve discoverability and maintenance, and integrate seamlessly with our existing development workflow.