Add initial mkdocs documentation website

[DaniruKun]

As discussed with @axelson on Slack, this is the first version of the ElixirLS documentation website built using Mkdocs

At the moment, it contains everything a user requires to learn how to use and install ElixirLS for any supported editor, as well as intuitive navigation, links and universal search.

The GitHub Actions for automatic publishing of the website to Github Pages will be added in a future PR, as well as more screenshots and GIFs showcasing the features.

At the moment, the website can be manually served using mkdocs serve (for development) and built using mkdocs build.

[axelson]

Thanks for this! The site will be a nice improvement for users.

And (as mentioned in Slack) I like keeping it all in one repository because then a PR can update the website as well.

As part of the this transition I'm thinking maybe should start using a dev branch, and only merge master when making a release. But I'll start another issue for that.

For this PR can you:

• add site to .gitignore
• write up some instructions on building the documentation website locally in DEVELOPMENT.md

Thanks!

[DaniruKun]

@axelson added the changes you requested. Regarding the automated publishing of changes to the site - it looks like there is a new Github Action for this that greatly simplifies this, should be very easy to setup: https://github.com/jamesIves/github-pages-deploy-action

[axelson]

That new GitHub action looks like it'll be good. Let's add it to .github/workflows/release-asset.yml as a new job

[DaniruKun]

@axelson updated with your comments. I set Github Actions to deploy the build website into the branch mkdocs. This is the one that you have to configure Github to present from.

[axelson]

Thanks again for this! ❤️

[DaniruKun]

I think next we can add the link to the site somewhere in the repo description, or maybe also in the org home. It might also be worth revisiting the README and removing a bunch of stuff that was migrated to the site anyway, and just leave a quick QuickStart guide and a link to the site.