Update version numbers (and related information) on Wiki

[c-lipka]

In preparation to moving forward with v3.8, the following pages of our documentation on the Wiki warrant updating, as they still refer to v3.7 (or even earlier version numbers):

• https://wiki.povray.org/content/Documentation:Tutorial_Section_5#Contributors_to_POV-Ray must be brought up to date for v3.8.
• https://wiki.povray.org/content/Documentation:Tutorial_Section_5.1#Linux.2FUnix: At best, the version number in file name will have to be fixed - at worst, the whole section may need a major overhaul.
• We may also want to update https://wiki.povray.org/content/Documentation:Tutorial_Section_3#Text_Object to use "POV-RAY v3.8" (sic! - make sure to include the v before the version number or I'll scratch out someone's eyes) as sample text.

Also, if anyone knows of any other piece of the official documentation that needs adapting to v3.8, please comment here.