Tweak command names

* sly-stickers.el (sly-stickers-fetch)
(sly-stickers-shortcut-mode-map): Use `sly-stickers-fetch'
(sly-stickers-fetch): Renamed back from `sly-stickers-fetch-all-and-forget'.
Update documentation and NEWS.md file

Ongoing work for issue #9.

* NEWS.md: Move sly-stickers description to the Texinfo
documentation. Add bugfix informatino.

* doc/sly.texi: "Contributed Packages" ->
"Extension". Re-add menus using emacs menu generator.
Remove vindex annotations.
(Loading Contribs): Rewrite.
(REPL): Remove SLIME REPL references.
(Editing Commands): Removed section
(Autodoc): Renamed from `sly-autodoc-mode'
(SLY Stickers): New section.
(Extensions): Reorganized.
(sly-fancy): Removed section.
Link to documentation in the README