v6: make GenericIncluder linkIndexAutotitle behavior the default

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.