feat: improve the preview script and related documentation (#222)

Improvements
  - enforce 'redirect' and 'no fetch sources' config except if explicitly
  requested
  - allow to build all components/branches using a new `use-all-repositories`
  option
  - allow to build a set of components and branches with new options
  (`use-multi-repositories` and `component-with-branches`)
  - add a dedicated bash preview script for local developments. Everything can
  now be done with the script so specific npm scripts have been removed from the
  package.json file
  - new bash script option to skip the doc generation and only generate the
  Antora preview playbook
  - improve the site title management logic
  - allow to use boolean options without having to pass 'true' as argument

Fixes
  - allow the bonita '7.10' branch build
  - make the 'fetch sources' option work

Readme updates
  - reorganize to put end-user documentation first
  - put local developments information first as this is what developers mainly
  use
  - provide more details about the `build-preview` bash script usage (for
  instance http server and asciinema)