New scraper: graphviz (also known as DOT language)

[denilsonsa]

Closes: https://trello.com/c/Jaa1vC24

This is my second scraper. It's almost in a good shape, but I left a TODO comment because I couldn't figure out how to make UrlScraper reliably scrape the website. I keep getting random errors.

Otherwise, it's ready for review.

---

If you’re adding a new scraper, please ensure that you have:

• Tested the scraper on a local copy of DevDocs
• Ensured that the docs are styled similarly to other docs on DevDocs

• Added these files to the public/icons/your_scraper_name/ directory:
  • 16.png: a 16×16 pixel icon for the doc
  • 16@2x.png: a 32×32 pixel icon for the doc
  • SOURCE: A text file containing the URL to the page the image can be found on or the URL of the original image itself

[denilsonsa]

If anyone can figure out a solution for the web scraper, feel free to modify this PR. It feels like 95% of the work is already done, and I appreciate if anyone else manages to contribute the remaining work.

[simon04]

Awesome, thank you!

[simon04]

I've implemented a very basic retry logic in 8e07071 and cbaedce when the error code is 0/500/501/502/503/504 to fix the sporadically occurring errors as the following:

ERROR:                                                                                                                                                                                                                                                                            
  https://www.graphviz.org/docs/attr-types/pointList/
  RuntimeError: Error status code (0): Error in the HTTP2 framing layer
    https://www.graphviz.org/docs/attr-types/pointList/
    
    

  /Users/simon/src/devdocs/lib/docs/core/scrapers/url_scraper.rb:49:in 'Docs::UrlScraper#process_response?'
  /Users/simon/src/devdocs/lib/docs/core/scraper.rb:158:in 'Docs::Scraper#handle_response'
  /Users/simon/src/devdocs/lib/docs/core/scraper.rb:77:in 'block in Docs::Scraper#build_pages'
  /Users/simon/src/devdocs/lib/docs/core/requester.rb:59:in 'block (2 levels) in Docs::Requester#handle_response'
  /Users/simon/src/devdocs/lib/docs/core/requester.rb:58:in 'Array#each'
  /Users/simon/src/devdocs/lib/docs/core/requester.rb:58:in 'block in Docs::Requester#handle_response'
  /Users/simon/src/devdocs/lib/docs/core/instrumentable.rb:15:in 'Docs::Instrumentable::Methods#instrument'
  /Users/simon/src/devdocs/lib/docs/core/requester.rb:57:in 'Docs::Requester#handle_response'
  /Users/simon/src/devdocs/lib/docs/core/requester.rb:18:in 'Docs::Requester.run'
  /Users/simon/src/devdocs/lib/docs/core/scrapers/url_scraper.rb:38:in 'Docs::UrlScraper#request_all'
  /Users/simon/src/devdocs/lib/docs/core/scraper.rb:76:in 'Docs::Scraper#build_pages'
  /Users/simon/src/devdocs/lib/docs/core/doc.rb:115:in 'block in Docs::Doc.store_pages'