Explore parallelisation for build

[eelcoj]

The build process is bottlenecked by sequential page rendering. Each page is currently rendered linear (one-by-one).

Using the parallel gem, you can allow multiple pages to be rendered concurrently. 🍾

The number of threads is auto-detected based on CPU cores via the parallel gem (can be configured via Perron.config.build_threads: name TBD).

Some stats 📈

Site	Pages	Sequential	Parallel	Improvement
Perron docs	95	~9s	~5s	~45%
Rails Designer	483	~33s	~23s	~30%

Benchmark (Rails Designer site, 483 pages):

Page rendering (sequential): 27.06s
Page rendering (parallel):   18.71s

If you have a medum plus-sized site (500+ pages), do add config.parallel_rendering = true to the Peron initializer to enable parallel rendering (it is off/false by default)

Some notes:

• Thread-safety: Rails controllers are thread-safe (no shared mutable state during page rendering)
• Warm runs: Improvements are most visible with cached assets (typical production scenario)

[eelcoj]

Failing CI expected; did not add parallel to the appraised gemfiles.

If someone is willing to share their benchmarks using this branch in the comments. ❤️

Am wondering if this PR can make a difference or if this approach is not right (am not too familiar with the internals of the the parallel gem—and in general this matter.