Hashtbl tutorial: compare `add` and `replace`

Fixes https://github.com/ocaml/ocaml.org/issues/452
Merge remote-tracking branch 'upstream/master'
Merge pull request #517 from garrigue/master

add abstracts to program
Fix links to CUFP slides (Verlaguet)
Merge pull request #525 from smondet/master

Fix links to CUFP slides (Verlaguet)
announce 4.02.0 release
fix link to portability.html

Were incorrectly pointing to deprecated caml.inria.fr page.
install.md: update OPAM install instructions

Addresses #526.
Fix typo in FFTW description
Merge pull request #529 from isms/typo

Fix typo in FFTW description
releases/4.02.md: fix extensible types notation

Reported by @whitequark.
releases/4.02: fix binary distribution info

Binary distributions no longer being provided by Inria, so links were
broken. Provide alternate links where available. Closes #528.
Removed duplicate word "first"
Merge pull request #530 from LuqiPan/patch-1

Removed duplicate word "first"
Add links to videos for OUD2014
Merge remote-tracking branch 'upstream/master'
add links to slides
also add html files from media