Update First Steps doc

[fpagnoux]

The Test the impact of a reform paragraph is obsolete.

We may want to redesign or remove the whole Getting started section.

[MattiSG]

I thought this would be done in #70. There is a whole revamp of the doc to be done, and I'm not 100% sure it can be properly scoped.

[MattiSG]

• [x] Drop section 2 (First steps) altogether (duplicate of section 5).
• [x] Rename section 5 (coding the legislation) to “From law to code”.
• [x] Move section 5 (coding the legislation) to section 2.
• [x] Merge section 3 (thinking in vectors) into section 5.3 (vector calculus limitations).
• [x] Move section 4 (publishing results) between sections 8 and 9.
• [x] Rename section 0 “About the project” to “Introduction”.
• [x] Move “slides” paragraph of section 0 to section 9 (community).
• [x] Remove “microsimulation” paragraph of section 0, add Wikipedia link when the term is first used.
• [x] Simplify and merge the “technical choices” paragraph of section 0 into the first paragraph.
• [x] Move the “Project history” paragraph of section 0 into section 9 (community).

Rider:

• [x] Add title: "OpenFisca" in book.json.