Add styled text (bold/color) example to jsPDF README (Hacktoberfest Contribution)

[KARLE-ATHARVA]

📄 Hacktoberfest Documentation Contribution

This PR adds a small but useful example demonstrating how to apply bold fonts and custom colors when generating text in jsPDF.

✅ Why this is helpful

• The README currently shows only basic doc.text() usage.
• Many developers search for "how to style text (bold, color, size)", so this improves onboarding.
• Example is non-breaking and consistent with existing format.

Looking forward to feedback from maintainers!