docs: move recipes to how-to guides

At the moment, many pages in "Behind the scenes" are mixing explanations with how-to guides. This makes it difficult for our users to find the relevant content (a recipe to achieve the outcome they want).

As part of our adoption of the four-quadrant documentation system (see #5549 and #5718), we should extract all guides from explanation pages into guide pages (see #5768 for inspiration) and keep explanation pages focused on explaining the concepts & providing background information.

This story is a part of #5113 Documentation improvements 2020Q3.

🎆 Hacktoberfest 2020

Greetings 👋 to everybody coming to this issue with the desire to help us improve our docs and score a pull request (or more?) in this year's Hacktoberfest challenge.

Here are few tips 👀 to make your start easier:

• Pick a task (a doc page) from the check-list below and leave a comment to let others know that you are going to work on it. Some of the tasks have a dedicated GitHub issue created, please check that issue for more details.
• Some doc pages have a lot of content, the amount of effort needed may feel intimidating. Don't worry! It's perfectly fine to pick just one section and convert it into a How-to guide. Baby steps FTW, every little helps!
• If you are new to GitHub pull requests, then you can learn about the process in Submitting a pull request to LoopBack 4.
• If this is your first contribution to LoopBack documentation, then read Documentation guide for LoopBack 4 developers please. Consider previewing loopback-next docs only for a faster feedback loop. Finally, please refer to Contributing to LoopBack documentation for a more advanced information on our Jekyll-based documentation infrastructure.
• Feel free to ask for help in #loopback-contributors channel, you can join our Slack workspace here.

📋 List of pages to rework

• Application --> #6106
• Server --> #6107
• Controllers --> #6108
• Models, "Custom Validation Rules and Error Messages", and especially "Defining a model at runtime"
• HasMany Relation
• BelongsTo Relation
• HasOne Relation
• DataSources
• Repositories
• Services
• Interceptors
• Life cycle observers
• Routes --> #6109
• Middleware-based Sequence for REST Server
• Routing requests
• Parsing requests --> #6110
• Middleware
• Using Express Middleware
• Mixins
• Binding

Choose one unchecked page from the checklist, such as Application.html, Controllers.html, or a relation page, and read it alongside the four-quadrant documentation guidance and issue #5768. Extract its how-to content into a guide page while keeping the original focused on concepts and background. Done means the selected page is reworked and its checklist item or dedicated issue is updated.