docs: switch fig links to github page - part 2

[yunjunz]

Description of proposed changes

• docs: switch the image src links from Yunjun's obsolete WordPress website to the GitHub page of insarlab/figs, for better independency. A continuation of demo/test: add SanFranBaySenD42 for GMTSAR #1229, docs: update fig links for demo datasets #1234, docs: switch fig links to github page #1237.

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.