scribble/html docs: clarify the effect of 'abs un `url-roots`
plt-web: change default site mode to 'abs

Using site-relative paths that are absolute within the site
avoids problems with files that are moved or used from multiple
contexts, such as "page-not-found.html".
plt-web: add `make-indexes`
meta/new-web: fill in corners

Use soft links to represent redirections on the web site, where
all redirections needed now are within a site (specifically, the
download site).

Adjust various pages that are essentially for backward
compatibility.