v6: make GenericIncluder linkIndexAutotitle behavior the default

[martyanovandrey]

Tracked in #1899.

Background

PR #1898 introduces a new opt-in option linkIndexAutotitle for the generic includer. When enabled together with linkIndex, the directory entry's title is taken from the H1 heading of the nested index.md (via the existing autotitle pipeline), instead of the folder name.

The flag defaults to false in v5 to preserve backward compatibility for existing linkIndex consumers (added in PR #1784) that rely on folder names for directory titles.

Proposal for v6

• Make the index.md autotitle behavior the default when linkIndex is enabled and autotitle is not disabled.
• Either remove the linkIndexAutotitle option entirely, or invert its meaning (e.g. useFolderNameForLinkIndex) — to be decided.

Why this is breaking

Existing linkIndex users would see their toc directory titles change from folder names to whatever the H1 of the linked index.md says. Without an explicit migration step, this is a visible behavior change.

Migration notes

• Document the behavior change in the v6 release notes.
• Suggest setting linkIndexAutotitle: false (or equivalent) in v5.x ahead of the upgrade for users who want to keep folder names.