Fix two missing '.'.

Reported by Laurent Orseau.
small improvements to style of examples on front page
Fix switch to small navbar.

Fix suggested by cosmez in #racket.
Fix jerkiness of navbar navigation.
Add links to the intro paragraph, unobtrusively.
Margin at the bottom.
Revise "get started" para.

Based on suggestion from Neil Van Dyke.
meta/new-web: fix blog stub
Add TAGS target to toplevel makefile. Adjust clean target to match.
Another makefile CFLAGS/CPPFLAGS adjustment

Missed another spot in c97184b581.
Make check-exn/check-not-exn keep the check failure message.
Fix tc-results->values and filter-tc-results to handle polydots.
racket/gui gtk: fix reference count on widget extracted in choice%

Incorrect referencing causes crashes on Kubuntu, for example.
allow 'contract' to use #f for the positive
name, but still disallow #f at the blame record
level

closes PR 14387
fix contract guide example
handle the case where the links file contains an absolute path

also try to make fewer assumptions about the content of links files
closes PR 14392
Make users of check-below call it with same kind arguments.
Make check-below only accept arguments of the same kind.
Raise a better message for ctc generation in TR

This provides a better explanation of contract generation
failures for module-provided bindings.
Revise Scribble docs for the new package system

Replaced mentions of PLaneT with the new package system
in sec 4.1 of the Scribble docs.