The GRASS GIS manual pages of the different versions have been published for a long time with a difficult to understand concept of being invisible, redirected or shown, which also strongly affects the search engine ranking.

SEO: Without indication of "canonical" URLs different versions wipe each out out in search engines. Canonical tags help consolidate duplicate or similar content by specifying the preferred version of a page, ensuring search engines index and rank the desired URL while avoiding duplicate content issues.

This PR changes the cronjob scripts to
- inject "grass-stable" as the "canonical" into older manual pages under versioned URL
- inject "grass-devel" as the "canonical" into the development manual pages under versioned URL

Like this no "duplicate content" from a SEO perspective should occur.

Also `robots.txt` is updated to reactivate the manual pages of old GRASS GIS versions (which now contain "grass-stable" as the canonical).

Fixes OSGeo/grass#4579

… to point to "stable" manual (rather than "devel")