Add docs/source/api to docs/source/index filemap AND add better versioning support

[hasezoey]

Summary

This PR consists of 2 parts:

part 1:

• add dos/source/api to the docs/source/index filemap
• move code from docs/source/splitApiDocs.js to /scripts/website.js
• add jsdoc types for docs/source/api out (/ module.exports.docs / data)
• rename api property name to title to match docs/source/index scheme
• only promisify pug.render once
• remove docs/source/splitApiDocs.js
• add watchers for all of documented lib/* files
• add watcher for docs/api_split.pug (reload all api files)
• update scripts/generateSearch for the changes made
• update scripts/generateSearch to use the already generated url instead of re-doing it
• add a log of how many files will be processed when running docs:generate

part 2:

• add better versioning support by making the process more automatic
• refactor how versions are parsed and from where (previously it was from CHANGELOG, but now its from git tag)
• add ability to deploy to sub-directy (like in gh-pages) directly like docs/6.x
• add dev-dependency fs-extra to copy the full directories (not using ncp because it is old and nodejs does not have a native way, would replace ncp later)

fixes #13148

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Notes on the new versioning:
the new version in made so that without environment variable DOCS_DEPLOY=true it will behave as if no versioning is present (not outputting any new sub-directories or adding things to urls), but once the environment variable is set, it will:

• if the latest version is detected, or as fallback; it will deploy to docs/ like always
• if a older version is detected, it will deploy to something like docs/6.x, including all files that are required (like js, css, and even <root>/index.pug)

this change makes it so that versioned docs can still be locally run without screwing up static paths (like js and css) but also easy to deploy to gh-pages (hopefully)

should i add a package.json script for deploying with DOCS_DEPLOY=true?

[vkarpov15]

This looks great 👍 I was just having some trouble testing docs generation on 6.x branch, looks like this might help with that.

[hasezoey]

@vkarpov15 would you want a backport of this PR to the 6.x branch?

[vkarpov15]

Yeah backport would be nice. Merge conflicts will be a bit messy, but hopefully not too bad.