Update OCaml meetings page with the next OUPS event
companies: add cacaoweb
companies: fix cacaoweb description
Fix 99problems "Gray code".

Fixes https://github.com/ocaml/ocaml.org/pull/492
Order more naturally the "Documentation" footer

Fixes https://github.com/ocaml/ocaml.org/issues/461#issuecomment-48292223
Translate "Install" to French
Use List.rev_map in "Gray code" (99 problems)

See https://github.com/ocaml/ocaml.org/commit/8f518cd6f30766dd18334ff1f48f2fd3439bc099#commitcomment-6929121
Added a solution for laying out binary trees

Also added the tree used in the examples
Removed unneeded alias, renamed parameter
Improve the presentation of "Layout a binary tree" (99 problems)
Add a solution to "Layout a binary tree" (99 problems)
Update index.md

fix: typo
Update index.fr.md
provide narrative for depth-first-search graph traversal
remove Core.Sequence from program