freeciv21-manual does not get images needed

[jwrober]

Describe the bug
If you call freeciv21-manual for a ruleset, it will write out the proper files. However it does not get the images needed for terrain, buildings, and wonders.

To Reproduce
Steps to reproduce the behavior:

1. Open a command terminal
2. run freeciv21-manual -r civ1
3. Open civ13.html and notice missing terrain icons. Same with civ14.html, and civ15.html.

Expected behavior
Image files are copied to an images directory and properly referenced.

Screenshots
N/A

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-alpha.1
• Ruleset/Longturn game (if applicable): N/A

Additional context
There isn't a "--tileset" option, so a default terrain would need to be set (such as Amplio2)

[lmoureaux]

How much do we want to support -manual vs pyciv's approach?

[jwrober]

-manual requires no other dependency and is shipped with the game. Maybe it should just be image-free in the HTML output? I don't want to get rid of it altogether I don't think.

[jwrober]

Coming back to this... I think an edit to freeciv21-manual to not attempt any images and just generate plain HTML is a easy approach and then we let pyciv do the bigger more "sexy" effort of getting images and generating the RST we need for docs.