Support parallel processing in site/collection generation

[ronaldtse]

When building a site you often want to compile multiple documents together since Metanorma is not parallelised.

This issue is to add a build queue that serves multiple concurrent processors so that a site can be built in minimal time. By default it should use 2 cores and allows more, e.g. metanorma config set build.parallel 4 for 4 processes.

[ronaldtse]

In response to @skalee 's comment in metanorma/coradoc#90 , in site processing we deal with multiple independent documents, so there is no issue with parallel processing as there is no shared state.