Add documentation for Free, Premium, and other add-on assessments to developer.yoast.com

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This task was created as part of this task: https://github.com/Yoast/non-dev-tasks/issues/39
See report in first comment to this task for context.

All assessments:

In developer/docs/features/analysis create a folder called assessments.

overview.md

Within this folder create one overview.md with some general information and links to all assessment specific docs. Could also contain a legend/explanation of the card system (see below).

This should have a docusaurus header like so:

---
id: overview
title: "Assessments: Overview"
sidebar_label: Overview
description: This documentation provides a list of all our assessments.
---

This overview should contain a list of all (Free and Premium) SEO, Readability and Inclusive Language assessments by their type/source. And all assessments should link to the assessment specific page (see below). Note: there are separate tasks to add documentation related to add-ons and Shopify:

• Add documentation for e-commerce specific assessments and scoring criteria to developer.yoast.com
• Add documentation for add-on specific assessments to developer.yoast.com

Example of how this will look:

SEO

• Keyphrase in introduction
• Keyphrase length
• ...

Readability

• Subheading distribution
• Paragraph length
• ...

Inclusive Language

• Age
• Appearance
• Culture
• ...

---

Assessment specific pages

Within the assessments folder, create a folder for each assessment type/source. NOTE: use snake casing as this appears to be the convention withing developer.yoast.com. For example inclusive-language.

In each respective folder, create an .md file for each assessment. (Again, use snake casing: keyphrase-in-introduction.md)

Each file should have a Docusaurus header at the top. For example:

---
id: KeyphraseLengthAssessment
title: "Assessments: KeyphraseLengthAssessment"
sidebar_label: Keyphrase Length
description: This documentation provides an overview of the Keyphrase Length Assessment.
---

Docusaurus will interpret this header.

Each file could contain a card with some essential information in a structural format. This could also house some information from SCORING SEO.md, SCORING READABILITY.md and SCORING INCLUSIVE LANGUAGE.md that is not in the second card below. For example:

Keyphrase length
Name In Code	KeyphraseLengthAssessment
Title URL:	https://yoa.st/33i (link placement is in bold in the feedback strings)
Call to action URL	https://yoa.st/33j (link placement is in bold in the feedback strings)
What it does	Checks whether the number of ( content ) words in the keyphrase is within the recommended limit. For languages with function word support only content words are considered. For languages without function word support all words are considered.
Uses synonyms	❌
When it applies	Always.
Addon/Premium	❌ <whether it is part of an addon or premium>
Languages	Agnostic
Cornerstone	❌ <whether it is changed by cornerstone toggle>
Uses highlighting	❌
Uses morphology in premium	❌
Other	<Any additional info that might be relevant.>

Below this card, create a card for scoring. This can be copied from SCORING SEO.md, SCORING READABILITY.md and SCORING INCLUSIVE LANGUAGE.md. For example

Bullet	Score	Criterion	Feedback
Red	-999	No focus keyword set	Keyphrase length: No focus keyphrase was set for this page. Set a focus keyphrase in order to calculate your SEO score.
Red	3	Keyphrase length > 8 words (> 9 for languages without function words support)	Keyphrase length: The keyphrase contains x (content) words. That's way more than the recommended maximum of 4 (content) words. Make it shorter!
Orange	6	Keyphrase length between 5-8 words (7-9 for languages without function words support)	Keyphrase length: The keyphrase contains x (content) words. That's more than the recommended maximum of 4 (content) words. Make it shorter!
Green	9	Keyphrase length between 1-4 words (1-6 or languages without function words support)	Keyphrase length: Good job!

Below this card there is space for free text/context/screenshots/desired behaviour.
Do not remove these cards from the yoastseo package. Also do not move any other documentation from the yoastseo package. There is a separate task to deal with that: https://github.com/Yoast/lingo-other-tasks/issues/211

Start by inspecting the existing structure under docs/features/analysis and the Docusaurus documentation conventions. Use the linked SCORING SEO.md, SCORING READABILITY.md, and SCORING INCLUSIVE LANGUAGE.md files as source material; done means adding assessments/overview.md plus the requested assessment-specific folders and pages without removing or moving yoastseo documentation.