docs: move recipes to how-to guides

At the moment, many pages in "Behind the scenes" are mixing [explanations](https://documentation.divio.com/explanation/) with [how-to guides](https://documentation.divio.com/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 :wave: 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](https://strongloop.github.io/strongloop.com/strongblog/2020-hacktoberfest/).

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](https://loopback.io/doc/en/lb4/submitting_a_pr.html).
- If this is your first contribution to LoopBack documentation, then read [Documentation guide for LoopBack 4 developers](https://loopback.io/doc/en/lb4/code-contrib-lb4.html#documentation) please. Consider [previewing loopback-next docs only](https://loopback.io/doc/en/lb4/code-contrib-lb4.html#previewing-loopback-next-docs-only) for a faster feedback loop. Finally, please refer to [Contributing to LoopBack documentation](https://loopback.io/doc/en/contrib/doc-contrib.html) 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](https://join.slack.com/t/loopbackio/shared_invite/zt-8lbow73r-SKAKz61Vdao~_rGf91pcsw).

## 📋 List of pages to rework

- [ ] [Application](https://loopback.io/doc/en/lb4/Application.html) --> #6106
- [x] [Server](https://loopback.io/doc/en/lb4/Server.html) --> #6107
- [ ] [Controllers](https://loopback.io/doc/en/lb4/Controllers.html) --> #6108
- [ ] [Models](https://loopback.io/doc/en/lb4/Model.html), "Custom Validation Rules and Error Messages", and especially "Defining a model at runtime"
- [ ] [HasMany Relation](https://loopback.io/doc/en/lb4/HasMany-relation.html)
- [ ] [BelongsTo Relation](https://loopback.io/doc/en/lb4/BelongsTo-relation.html)
- [ ] [HasOne Relation](https://loopback.io/doc/en/lb4/HasOne-relation.html)
- [ ] [DataSources](https://loopback.io/doc/en/lb4/DataSources.html)
- [ ] [Repositories](https://loopback.io/doc/en/lb4/Repositories.html)
- [ ] [Services](https://loopback.io/doc/en/lb4/Services.html)
- [ ] [Interceptors](https://loopback.io/doc/en/lb4/Interceptors.html)
- [ ] [Life cycle observers](https://loopback.io/doc/en/lb4/Life-cycle.html)
- [x] [Routes](https://loopback.io/doc/en/lb4/Routes.html) --> #6109
- [ ] [Middleware-based Sequence for REST Server](https://loopback.io/doc/en/lb4/REST-middleware-sequence.html)
- [ ] [Routing requests](https://loopback.io/doc/en/lb4/Routing-requests.html)
- [x] [Parsing requests](https://loopback.io/doc/en/lb4/Parsing-requests.html) --> #6110
- [ ] [Middleware](https://loopback.io/doc/en/lb4/Middleware.html)
- [ ] [Using Express Middleware](https://loopback.io/doc/en/lb4/Express-middleware.html)
- [ ] [Mixins](https://loopback.io/doc/en/lb4/Mixin.html)
- [ ] [Binding](https://loopback.io/doc/en/lb4/Binding.html)

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.

Written by the indexing model from the issue text.