partially deal with #321

this replaces ```tryocaml -> ```ocaml in many places
Fix conversion failures to HTML.

The concerned pages now render well to HTML instead of displaying an
empty page.
Improve the layout of the top of the "Learn" page.

Partially addresses https://github.com/ocaml/ocaml.org/issues/280
Put the 2 book covers next to each other for all page widths
Update the README instructions to the new make targets.
Typo.
learn/index.md: reword intro text

Addresses #280.
Add vertical space between ruled <h1> and <pre>, <ol>.

Partially addresses https://github.com/ocaml/ocaml.org/issues/280
Format tutorial: Make the code belong to the items

Partially address https://github.com/ocaml/ocaml.org/issues/321
once the omd bugs https://github.com/pw374/omd/issues/60
and https://github.com/pw374/omd/issues/61 are fixed.
companies.md: fix Incubaid logo link

We had the img already. It just wasn’t linked to.
companies.md: add Psellos logo
Merge pull request #328 from agarwal/master

fix company logos
consistently link section titles on landing pages

IIRC @dbuenzli mentioned this problem, but I can’t find where.
Update README.md
Move to bootstrap_mod.css the bootstrap corrections.
Decrease the vertical space between <p> and <pre>
Fix a remaining error in the "Format" tutorial.

With commit 3885a9cb94cc3d41bad626457213922910d3ae64, it fixes issue
https://github.com/ocaml/ocaml.org/issues/321
Fix layout + errors in the French "Format" tutorial.
Transform some white space into unbreakable space
Link the title "A taste of OCaml" on the front page

This complements commit 398eb71441f53fd619e50f3a9335e8588707f570