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

Chọn một trang chưa được đánh dấu trong checklist, chẳng hạn như Application.html, Controllers.html hoặc một trang về quan hệ, rồi đọc trang đó cùng với hướng dẫn tài liệu bốn góc phần tư và issue #5768. Trích xuất nội dung how-to của trang thành một trang hướng dẫn, đồng thời giữ cho trang gốc tập trung vào các khái niệm và bối cảnh. Công việc được xem là hoàn tất khi trang đã chọn được làm lại và mục tương ứng trong checklist hoặc issue riêng đã được cập nhật.