Revise skin instructions, move to separate README

[egmontkob]

Is there an existing request for this?

• I have searched the existing issues

Is your feature request related to a problem?

Currently the colors that can be used in skins, as well as how inheritance works, are described in sand256.ini, the first 256-color skin.

Now that we have many 256-color skins, it's hard to find this info.

It should be moved to a separate README file which also gets installed to the skin directory. That's how people can easily find it. Or to the manual page?

While at it, do some further cleanups and fixes:

• mention that from [whatever-section]/_default_ we further fall back to [core]/_default_.
• merge truecolor syntax description from the seasons* skins.
• Wrong comment about 256 colors in truecolor skins #4885
• mention the 256colors and truecolors keywords
• remove sentences referring to ancient versions, like sand256: needs ncurses or slang from 10 years ago or newer; modar*: needs mc 4.8 or newer
• mention that the file needs to be encoded in UTF-8 (all those *-char definitions)
• mention that currently only narrow (single charcell) characters are supported for *-char
• point to the online skin viewer, to make it easier to discover

@horkykuba as per #4885 you have ongoing work hereabouts, I don't want to conflict with that so I'll work on this once you're done with that. Or, if you feel like taking on any of these, I won't hold you back :)

Describe the solution you'd like

.

Describe alternatives you've considered

No response

Additional context

No response

[horkykuba]

@egmontkob Feel free to do any changes. The only change currently in my queue that's related to this is:

diff --git a/misc/skins/seasons-autumn16M.ini b/misc/skins/seasons-autumn16M.ini
index 665395508..98fcf5e8b 100644
--- a/misc/skins/seasons-autumn16M.ini
+++ b/misc/skins/seasons-autumn16M.ini
@@ -5,8 +5,8 @@
 # True colors are referred to by the standard #rrggbb or #rgb notation, the
 # short form is interpreted by doubling each hex digit.
 #
-# In addition to these, you can use the basic 8 (non-light) colors and
-# the attributes. However, you can't use the 256 indexed colors.
+# In addition to these the 256 palette colors are still available, and
+# obviously so are the attributes. See the sand256.ini skin file for details.