doc: rework installation/setup guides, general refactoring

[nodiscc]

A full and concise installation guide with examples is a frequent request.

The documentation should provide such a guide for basic installation needs, while explaining alternative/advanced configuration at the end of teh document.

Links to reference guides and documentation should be used more frequently to avoid recommending an outdated or excessively complex configuration.

Ideally the download, installation, server setup and security sections should fit on a single page.

Another page can walk the user through installing Shaarli in Docker behind a reverse proxy.

• Fixes [Install Issue] 403 Forbidden #1381
• Fixes [Question] Example for independent nginx deployment #1455

[nodiscc]

Still work in progress, see:

• https://shaarli.readthedocs.io/en/doc-rework-setup/Server-configuration/ (currently redoing/condensing the nginx section)
• https://shaarli.readthedocs.io/en/doc-rework-setup/Download-and-Installation/
• https://shaarli.readthedocs.io/en/doc-rework-setup/Release-Shaarli/

As you can see this PR also proposes dropping support for the stable branch. I think the current model is complex/confusing, it adds maintenance and has no great benefits. Under the proposed model you can:

• Install the latest tag/release using any available method (release zip, git clone/zip + composer). Upgrades to the next release must be done manually (overwrite with new zip contents, or git remote update; git checkout new_tag; composer ...)
• Install the release branch from git clone/zip (the only benefit git remote update; git checkout is simply replaced with git pull), or the master branch using the same method.

In short the release branch would now be considered as stable (in its current form it's kind of like an oldstable branch.). What do you think?

[k4j8]

In doc/md/Server-configuration.md, I had to have the Directory section outside of VirtualHost rather than within it, but it may be my configuration (see #1423). I also had to change the configuration within Directory as a result of running a newer version of Apache. See http://httpd.apache.org/docs/current/upgrading.html.

I'm running Arch Linux and have only had my webserver for a few months. If it works for you, feel free to ignore.

[nodiscc]

I will definitely re-test all setup procedures before removing the work-in-progress status. Apache 2.4 will be used for the test. <Directory> should work inside a virtualhost definition but I will double-check everything, thanks

[nodiscc]

Rebased, squashed.

• See the commit message for details
• See the diff
• Seethe rendered documentation
• Feel free to ask about anything

I still have to test some procedures and change ordering (for example I know the setup procedure still takes you back and forth between 2 pages), but the bulk of it is done. Let me know what you think.

[ArthurHoaro]

Impressive work!
I did not reread everything as it's quite hard to follow the moved part, but the result you linked looks great to me.

[nodiscc]

I solved my NAT problems which were preventing me from testing install procedures with proper Let's Encrypt certificate generation. I will do another pass this weekend and probably include some asccinema demos of the process.

[nodiscc]

rebased.