build,doc: improve construction of the website
doc: improve documentation

Some stuff was made:

- fix the README
- move the configuration-file section to its own page
build,doc: create website for a given git branch
test: complexify the `website` workflow
build,doc: start converting some OCaml+MD to HTML
test,doc: improve documentation of `cli.ml`
doc: add test about the configuration file
build,doc: create HTML manpages for sub-commands
doc: improve the *Ketrew* section of the README
doc: improve the “code” CSS
build,doc: improve links to `ml` and `mli` files
doc: add a page about plugins
doc: document test `dummy_plugin.ml`
doc: add documentation to `dummy_plugin_user.ml`
doc,build: improve the titles of pages
test,doc: add the `preconfigured_main.ml` example
doc: add the `Alternative_CLI_Application` page
doc: improve `Long-Running_Plugins` page
doc: fix and improve the `Ketrew_daemonize` module
doc: fix internal links for Github view