ci: Optimize `Deploy Site` workflow

[wpiet]

Add a step to check if site content has changed, preventing unnecessary GitHub Actions execution. Content changes are now detected based on modifications to the following file types:

• .html,
• .css,
• .js,
• .png,
• .md,
• .rst.

[wpiet]

Changing to a draft, as it would be good to implement better handling of constantly changing sitemap.xml files (due to timestamps being updated to current date whenever build script is invoked).