[docs] Documentation addition for improvements to allowed characters on XY chart axis labels (Issue #7157, PR #7224)

[relturu]

Mermaid’s XY chart parser treats axis labels as ‘ASCII-only’ tokens, so labels with accents cause Unrecognized Text errors. A recent PR adds support for Unicode letters, allowing accents and international characters in unquoted labels. Documentation should be updated in xyChart.md to explain this new behavior.

In association with Issue #7157 and PR #7224.

[relturu]

I would like to be assigned this issue as my colleagues are working on the PR to fix the issue.