Add DIR_DOC_HTML tree variable

[jidanni]

$ gmt docs

Only uses compile time paths,

$ strace gmt docs -Q end 2>&1 |fgrep end.html | nl
     1  access("/usr/share/gmt/doc/html/end.html", R_OK) = -1 ENOENT (No such file or directory)
     2  access("/usr/share/doc/gmt/html/end.html", R_OK) = -1 ENOENT (No such file or directory)
     3  access("/usr/share/gmt/doc/html/end.html", R_OK) = -1 ENOENT (No such file or directory)
     4  write(1, "https://docs.generic-mapping-too"..., 52https://docs.generic-mapping-tools.org/6.4/end.html

It should also be overrideable a new variable, e.g., DIR_DOC_HTML,
that the user could set on the fly to e.g.,
$HOME/Downloads/gmt-.../doc_release/html .

Else users will say

#7597 (comment) worked, now why can't I use the gmt docs command to somehow read it?

The new variable should be mentioned on
$ gmt docs

P.S., On line 4 above the program is looking in the same place as line 2
again.