fix broken template processing in jsdoc output

[boneskull]

because the files generated by jsdoc were output to docs/_site, they were not processed as liquid templates, which meant the custom reporter tutorial was broken (the included "simple reporter" fixture file was not loaded, and the raw template tag was output instead).

to fix this, we needed to:

1. change the output directory of jsdoc to docs/api (just somewhere else)
2. add this to .gitignore, obviously
3. tell eleventy to ignore .gitignore (just rely on its .eleventyignore file)
4. tell eleventy to pass-through all assets underneath docs/api (css, images, etc)
5. update the file-loading 11ty data script to output a raw file
6. update the tutorial to use a fenced code block (jsdoc markdown plugin seems to strip html?) and tell Prettier to ignore the liquid template directive

also:

• removed link to google group
• prettier-related reformats
• tweak data script to use fs.promises

(I lost like 30m on the .gitignore thing. that's surprising behavior on 11ty's part, IMO)

[coveralls]

Coverage remained the same at 93.932% when pulling 9b2fb0a on boneskull/fix-custom-reporter-tutorial into 02bdb6b on master.

[boneskull]

Ref: needs mochajs/mocha-docdash#6

[boneskull]

That update to docdash fixes the URLs. 11ty wants to rename all of the files from filename.html to filename/index.html, but jsdoc's links all use filename.html, so all the links were broken (as rightfully caught by the link checker).

The change in docdash should be a major bump since it throws front matter into all of the .html files.

[boneskull]

I have some "experimental" changes to the workflow here which:

1. Avoids re-downloading a bunch of images if the supporters images are already on disk
2. Does not fail if we cannot download a supporter image
3. Using npm start docs.dev (renamed from docs.watch--an alias for eleventy --serve), 11ty watches and executes jsdoc when source files change (and avoids this if they do not change, but e.g., docs/index.md does)

I'll open a PR for that stuff after this one lands

[boneskull]

This isn't urgent, so I can wait a bit longer to merge. cc @mochajs/core

[craigtaub]

Nice one.
I always found docs/api vs docs/_site/api annoying. Didnt even realise https://mochajs.org/api/tutorial-custom-reporter.html existed.