Describe the bug
There are notable SEO problems on mobile devices with the GRASS GIS manual pages.

• Text too small to read
  • see: https://support.google.com/webmasters/answer/9063469#small_font_size
• Clickable elements too close together
  • see: https://support.google.com/webmasters/answer/9063469#touch_elements_too_close
• Viewport not set
  • see: https://support.google.com/webmasters/answer/9063469#viewport_not_configured
• Content wider than screen
  • see: https://support.google.com/webmasters/answer/9063469#content_not_sized_to_viewport

Random example:
https://search.google.com/test/mobile-friendly/result?id=nIc8LtrwxGgKXYAouSsizw&hl=en
--> https://grass.osgeo.org/grass82/manuals/r.walk.html

Expected behavior

The manual pages should be responsive, i.e. mobile friendly.

Proposed solution for "Viewport not set"

It appears that fixing "Viewport not set" seems solvable by simply adding the next line to the page header:

<meta name="viewport" content="width=device-width, initial-scale=1">

which is easy to do here as in an additional line 42:

Result of a test

Current state without viewport:

• content not readable

Potential fix with viewport:

• content readable
• menu is gone (but not really useful anyway on narrow mobile devices)

If the viewport meta line makes sense I can open a PR.