CN: Standardize Documentation Formatting and Navigation for MkDocs

This merge request improves the Core Network documentation to align with MkDocs requirements.

The following guidelines have been applied:

Guidelines Followed

Fix Broken Links and Paths

• Updated all links with proper paths to ensure correct rendering in MkDocs.
• Links to images and results have been updated to reflect the correct paths.

Correct Headings

• Removed multiple H1 headings (#).
• Subheadings use H2–H6 (## to ######) for consistent structure.
• Fixed heading issues that affected the table of contents.
• Remove any trailing # after the headings.

Proper URL Formatting

• All URLs now follow standard Markdown syntax, e.g., [Example](https://www.example.com).

Proper Syntax

• Tables, lists, and code blocks use standard Markdown formatting to ensure correct rendering.

Linking to Files in the Same Folder

• Simplified internal links by removing unnecessary ../ when referencing files within the same directory.

• For example:

  [System Requirements](system_requirements.md)

Remove HTML Formatting

• Removed unnecessary inline HTML styling from documents, retaining only the content. This also applies for styling for references to figures.

Notes, Cautions and Warnings

• Converted notes, cautions and warnings into blockquote syntax (>).

This ensures the documentation is properly structured, consistent, and MkDocs-friendly.

To-do:

• Add files for individual network functions like docs/network-functions/amf/*

Reviewer: Additional checks:

• Verify the changes of the documents.
• Determine if we need to add a separate docs/README.md.
• Decide where to add documentation (if needed) on contributing to the documents: docs/README.md or a separate DOCS_GUIDELINES.md (or equivalent) file.