Replace JavaScript link fix with custom plugin

[thaJeztah]

This is a follow-up to #10549

This replaces the JavaScript link fix script with a custom plugin, based on the jekyll-relative-link plugin, and modified so that it can be used as Liquid "filter".

While it borrows from the jekyll-relative-links plugin, it takes some shortcuts;

• We use the code from jekyll-relative-links plugin to find/extract links on the page
• Relative links are converted to absolute links, using the path of the markdown source file that's passed as argument
• After conversion to an absolute link, we strip the ".md" extension; no attempt is made to resolve the file that's linked to. This is different from the jekyll-relative-links plugin, which does resolve the linked file. This functionality could be added in future by someone who has more experience with Ruby.

[thaJeztah]

@usha-mandya @StefanScherer PTAL

This is my first time writing anything in Ruby, so it's mostly "copy/paste" and "code-by-numbers" using a good amount of StackOverflow. I tried building the docs locally, verified that pages such as docker service create http://localhost:4000/engine/reference/commandline/service_create/ have correct links to docker service update http://localhost:4000/engine/reference/commandline/service_update/, and that "absolute" links to other sections also still work

[thaJeztah]

oh, interesting;

#16 78.45   Liquid Exception: undefined method `gsub' for nil:NilClass in engine/reference/commandline/deploy.md
221
#16 78.45 jekyll 3.8.5 | Error:  undefined method `gsub' for nil:NilClass

The deploy.md is no longer published (it was the deprecated/experimental top-level deploy). Let me check why it crops up here

[thaJeztah]

Opened #10715 to fix the error merged

[netlify]

Deploy preview for docsdocker ready!

Built with commit 9cab419

https://deploy-preview-10714--docsdocker.netlify.app

[usha-mandya]

Tested some random links and they all work fine.

[usha-mandya]

Not merging it yet to give Stefan a chance to review the Ruby updates.

[StefanScherer]

LGTM

I only saw missing links to the API versions 1.24, 1.29, ..., but that's probably because the Netlify preview does not have all pages in it.