Restructure the documentation

[lildude]

Description

The README file has grown to massive proportions making it quite daunting to read and harder to maintain.

This PR restructures the documentation to move most of the content out of the README file and move it to individual files withing a /docs directory.

As part of this re-org I've also taken the opportunity to discourage the use of the macOS-supplied version of Ruby due to the problems it causes when installing charlock-holmes.

I've also expanded on the releasing docs for GitHub staff.

[Rest of the template has been removed as it doesn't apply]

[Alhadis]

I have conflicting feelings here. On one hand, this strikes me as the sensible, natural thing to do from both a maintenance and reader perspective.

On the other… well, I've seen a lot of folks deep-link to specific sections of Linguist's readme (both in discussions, docs, and commit logs), which means this change will introduce bit-rot. 😕 Moreover, there are tricks people use to make huge, single-page files more accessible and easier to navigate:

1. Tables of contents linking to each heading.
   When it starts getting too lengthy, break it into columns.
2. “Back to top” links.
   I personally find them bothersome, but I see these a lot, so it's clear many folks like 'em/
3. Using <details> or <details open> to implement collapsible sections.
   This might work well for the Troubleshooting section, since impatient readers will likely be skimming each subsection looking for their relevant problem.



[lildude]

On the other… well, I've seen a lot of folks deep-link to specific sections of Linguist's readme (both in discussions, docs, and commit logs), which means this change will introduce bit-rot. 😕

I did think about this but I thought his was a small price to pay to properly segregate "function" from "support", hence I've included the very small docs ToC at the top of the README. This could be extended.

Moreover, there are tricks people use to make huge, single-page files more accessible and easier to navigate:

Indeed, but that introduces more complexity and doesn't help with the long-term maintenance or expansion of the docs.

[Alhadis]

I did think about this but I thought his was a small price to pay to properly segregate "function" from "support"

Now that you mention it, I never noticed how sharply divided the readme's content was.

👍 then.