Prepare 15.0.0 docs

[jeddy3]

Which issue, if any, is this issue related to?

Closes #6409

Is there anything in the PR that needs further explanation?

We are introducing some significant changes in 15.0.0, mainly deprecating rules and removing the processor config property. Many people will likely read the migration guide looking to understand how Stylelint fits into their modern toolchain. It's a golden opportunity for us to address some misconceptions and let people know what Stylelint is capable of and the direction we're taking.

This pull request makes changes to communicate that:

• linters and pretty-printers are complementary tools
• it's best to extend our standard config
• there are many useful rules not turned on in the standard config

It's difficult to make these types of content changes in isolation, and I needed to look at the whole picture. As such, the pull request is quite large, but I'll add notes to explain some of the bigger changes.

These changes are for:

• new users starting for the homepage
• existing users starting from the migration guide

The new customisation guide bridges the gap between our getting started guide (which sets our users up with the bare minimum) and our comprehensive but dense documentation (i.e. lists of rules, options and configuration properties). The guide highlights:

• a few of the capabilities of the rules not turned on the in the standard config
• that there are custom rules that can be used via shared configs or plugins directly
• a few useful options like --fix and `--cache

It acts as a gateway for users to explore further and it’s not meant to be comprehensive. It highlights some of the customisations that our users may like to try out first and hints at what Stylelint is capable of as a linter.

It also removes lists of 3rd party things into Awesome Stylelint . It was previously arbitrary what lists we had in the main docs and what we put in awesome Stylelint; this led to integrations having more prominence than plugins in our docs. 3rd party things are still mentioned in examples to provide context, though.

Other changes were made to make the docs consistent in tone, terminology, structure and conciseness.

Also fixes issues picked up by Grammarly while I was reading through all the pages.

(I've got a few days of issues and pull requests to catch up, which I hope to do over the coming days...)

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[jeddy3]

Thanks for the reviews, everyone. I've accepted the suggestions and made commits for the other pieces of feedback.

@ybiquitous A big thank you for all the catches. You spotted lots!

Any more feedback or can we merge?

[ybiquitous]

@jeddy3 Thank you so much for the many document updates! LGTM 👍🏼