Support multi-document YAML without forcing explicit start markers or removing separators

Description

We're struggling to use yamlfix when there are multiple YAML documents in a single file, but we don't want to enforce an explicit start marker at the top.

Example scenario:

book_library:
  - title: Why we sleep
    author: Matthew Walker
  - title: Harry Potter and the Methods of Rationality
    author: Eliezer Yudkowsky
---
book_library:
  - title: Why We Matter
    author: Emeka Anyaoku
  - title: Harry Potter and the Goblet of Fire
    author: J.K. Rowling
---
book_library:
  - title: No Sleep Till Brooklyn
    author: Beastie Boys
  - title: Harry Potter and the Half-Blood Prince
    author: J.K. Rowling

Current behavior:

• With explicit_start = true, yamlfix adds --- at the start and end of the file, which we do not want.

---
book_library:
  - title: Why we sleep
    author: Matthew Walker
  - title: Harry Potter and the Methods of Rationality
    author: Eliezer Yudkowsky
---
book_library:
  - title: Why We Matter
    author: Emeka Anyaoku
  - title: Harry Potter and the Goblet of Fire
    author: J.K. Rowling
---
book_library:
  - title: No Sleep Till Brooklyn
    author: Beastie Boys
  - title: Harry Potter and the Half-Blood Prince
    author: J.K. Rowling

• With explicit_start = false, all --- markers are removed, causing key duplication and breaking multi-document structure.

book_library:
  - title: Why we sleep
    author: Matthew Walker
  - title: Harry Potter and the Methods of Rationality
    author: Eliezer Yudkowsky
book_library:
  - title: Why We Matter
    author: Emeka Anyaoku
  - title: Harry Potter and the Goblet of Fire
    author: J.K. Rowling
book_library:
  - title: No Sleep Till Brooklyn
    author: Beastie Boys
  - title: Harry Potter and the Half-Blood Prince
    author: J.K. Rowling

Expected Behavior

Provide an option to:

• Preserve existing --- markers between documents.
• Avoid adding --- at the end of the file.
• Optionally allow a single --- at the start (front matter) only if requested.

Possible Solution

Introduce a new configuration option, for example:

# Preserve multi-document separators but do not add trailing markers
document_markers = "preserve"   # keeps existing '---' between documents, no extra at end

# Or allow explicit modes:
document_markers = "front_matter_only"  # adds one '---' at start, preserves others
document_markers = "remove_all"         # removes all markers

[ccoVeille]

I feel like the issue you have might be related to one thing you report.

For me, it doesn't make sense to have no documents separator between documents when explicit_start = false

As you stated, it leads to invalid YAML.

It doesn't mean the idea of multiple option is not something to consider, but the bug should be fixed.

Note: I don't see a usage for remove_all flag