Update links to HTTPS and add image alt text

[Sip4818]

This PR makes a few small but meaningful documentation improvements:

Updated insecure HTTP links to HTTPS

All the referenced websites support HTTPS, so upgrading the URLs improves security, prevents mixed-content warnings, and aligns with modern standards.

Added missing alt text for images

Improves accessibility for screen readers and helps follow Sphinx documentation best practices.

These changes do not modify any functional content, only improve the quality and accessibility of the documentation.