docs: move "Parsing requests" recipes to how-to guides

[bajtos]

At the moment, the Parsing requests page in "Behind the scenes" is 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 Epic #5783 move recipes to how-to guides.

Acceptance criteria

Move the following recipes into "How-to guides", decide yourself whether to create a new page for each recipe or group multiple recipes in one page.

• How to customize validation error messages
• How to configure parser options, e.g set limit on query string size and tweak body-parser config - see the second part of https://loopback.io/doc/en/lb4/Parsing-requests.html#custom-error-messages
• How to use a custom body parser in a controller method
• How to localize validation errors

Consider adding a "Common tasks" section to "Application" page, it should list pointers to new how-to guides. See Server >> Common tasks for inspiration.