Only create ToC for headings with links

What does this MR do and why?

Only create ToC for headings with links

If a heading in a file exists without a link inside it, creating items for the table of contents will through an error. A filter on headers ensures that only those elements that do have an ancore tag are added to the array. Affecting rich viewer render of AsciiDoc, reStructuredText.

Changelog: fixed

References

• Investigation into errors found in Sentry

Screenshots or screen recordings

Before	After
dropdown not even rendered

How to set up and validate locally

1. Go to a Project/Repository
2. Click plus sign in the header and choose New file
3. Create an example AsciiDoc file
4. Go to a file and make sure that a dropdown for table of contents renders correctly.

Example file:

// Test.adoc
= Test Code
Jane Smith <jane@example.com>
:description: test article
:doctype:  article
:revdate:  12025-08-19
:doctype:  article
:hardbreaks-option:


// ================================================================================
== title 1
// ================================================================================
* here is some content




// ================================================================================
== title 2
// ================================================================================
* first list item

* second list item
	** with nested item




// ================================================================================
== title 3
// ================================================================================
* some extra text

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Related to #539276 (closed)