docs: move recipes to how-to guides

まだ誰も着手していません。

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)

チェックリストから、Application.html、Controllers.html、またはリレーションページなどの未チェックのページを1つ選び、4象限のドキュメントガイダンスおよび issue #5768 と併せて読みます。元のページは概念と背景に重点を置いたまま、その how-to の内容をガイドページに抽出します。選択したページを作り直し、そのチェックリスト項目または専用の issue を更新すれば完了です。

索引モデルが issue の本文から書いたものです。