best-practices: best practices guide landing page
build: commenting out svg-pdf conversion lines until that is fixed
Adds placeholder files for image and coding manual

- coding manual will need to be replaced with an actual example
- awesome-placeholder with appropriate images (and updated references)
best-practices: adds link to best practices landing
Major additions to best practices section

- Adds content to landing page,
- Adds an intro to coding page that discusses general principles to bear in mind,
- Adds the first steps page, which details the process of starting to work with behavioral data,
- Adds holding pages so the build doest break for the in depth guides landing page and coding manual page.