Add usage documentation

[lebouquetin]

Syntax for content pages:

--- METADATA (first) ---
metadata value
--- DATA (second JSON Structure) ---
{}
--- BODY (last / third) ---
any content

metadata syntax --- METADATA (first) ---:

<name> <value> where:

• name is a string without any space char
• value is separated from name by a set of space chars (space, tab, etc)

Example: og:title Titre utilisé pour les métadonnées opengraph

Specific cases:

• comments for lines starting by a #
• blank lines: ignored

Data syntax --- DATA (second JSON Structure) ---:

The data section is interpreted as a large JSON-like string. Two exceptions in the "parsing":

• lines starting with a # are ignored, considered being a comment
• blank lines are ignored

Body syntax --- BODY (last / third) ---:

A standard HTML or Markdown content. HTML content may include jinja template stuff, like macros, blocks, etc

Note: the template engine used in the body section can be parameterized through the template_engine metadata (value to be set to jinja2 or django

[ClmntBcqt]

Merged in #30