[3.4] Fixed everything found by the spellcheck builder

[wouterj]

This completes what I started in #12610

I've also committed the custom word list and Sphinx configuration. This way, everyone is able to run it (and I accidentially removed the word list after the last time, so I don't want to loose it again).

Some quick notes:

• Many titles that consisted of option or function names are transformed to be a literal. This is supported by Sphinx and all references still work. On symfony.com code elements in a title are ignored and titles are displayed as normal (see https://symfony.com/doc/current/components/filesystem for an example)
• I once again updated many cases of library names. I used the main website/documentation of each library to find the proper casing (e.g. NGINX, webpack, npm and HInclude.js).

To avoid merge conflicts, I will merge this PR asap. Don't worry about the merge conflicts: I'll resolve them once and then they should no longer occur

[javiereguiluz]

Wouter, thanks a lot for this huge effort!

[wouterj]

@javiereguiluz this PR introduces some usage of the .. spelling:: directive that is provided by the sphinxcontrib-spelling package python package. Using it should be as simple as installing this package using pip. Are we able to install this package on the symfony.com server? (requires apt install enchant and pip install sphinxcontrib-spelling==4.2.0 pyenchant, we have to have v4.2.0 of the spelling package).

An alternative solution would be to remove this directives and add the remaining spellings to the wordlist and comment out the spelling configuration. But I'm not a huge fan of that (i.e. plaintext is correct in Security concept, but wrong in e.g. Console documentation).

[javiereguiluz]

@wouterj about the .. spelling directive ... I don't know what to say yet. I'm not sure if we could install this package ... and I don't know if this could impact @weaverryan's new build tool.

[wouterj]

For the new build tool, it would be a matter of creating the PHP class for this directive. Nothing needs to be rendered, so we just need to have the class to not have the tool fail on not knowing the directive.

However, the easiest is to just add them to the global word list. I'll make that change shortly :)