Fix navigation: level-based chunking, nested toc.yml, discrete sections, navigation_title

[Mpdreamz]

Summary

Stacked on #3802. Fixes the converted navigation so it matches the live elastic.co/guide sidebar exactly.

Root causes:

• Every included file became a page (IsIncludeRoot-only rule), producing ~2065 flat pages with 27 slug collisions instead of the correct ~2085 hierarchically nested pages.
• TocEntry had no Children and WriteTocYaml was non-recursive, so parent/child structure was never written to toc.yml.
• Sections from separate includes (e.g. migration_intro.asciidoc + individual migrate_8_N.asciidoc files) were assembled flat in ProcessInclude, losing AsciiDoc's natural level-based nesting.

Fixes:

File	Change
PageChunker.cs	Complete rewrite: single Traverse pass producing a tree of PageOutput. IsPage = !IsDiscrete && (Level==0 ? IsIncludeRoot : Level <= chunkLevel+1). Book root detection (single L0 child → index.md). Duplicate slugs get _2/_3 suffixes.
SectionNode.cs	Added IsDiscrete (from [discrete]/[float] block attribute).
AsciidocParser.cs	Passes pendingBlockAttr into ParseSection for IsDiscrete. Added NestSectionsByLevel() at the end of ProcessInclude — sections at level N following a section at level < N in the flat result list are re-nested as children, replicating AsciiDoc's hierarchical nesting (fixes the Migration guide pattern).
MarkdownEmitter.cs	Added HeadingLevelBase + UpdateHeadingBase so child headings rebase relative to the page root.
YamlWriter.cs	TocEntry gains Children; WriteTocYaml is now recursive.
ConvertCommand.cs	WritePages recurses page.Children; emits navigation_title frontmatter.
ChunkerTests.cs	New: 9 tests covering discrete sections, deep-section inlining, auto-id, duplicate slug suffixing, navigation_title frontmatter, cross-include level nesting, and nested toc.yml output.

Results for ES 8.19 (chunk:1):

• 2085 pages in a properly nested toc.yml (was 2065 flat with 27 collisions)
• No duplicate toc entries
• breaking-changes.md (= Migration guide) has migrating-8.0 – migrating-8.19 as children
• getting-started.md has navigation_title: "Basics: Index and search using APIs"
• quickstart.md has ## Requirements inline (discrete sections stay on-page)
• targz.md contains ## Next steps inline (Level 3 rebased under Level 2 page)

Test plan

• dotnet test tests/Elastic.LegacyDocs.Migration.Tests/ — 44 tests pass
• ./build.sh unit-test — 314 tests pass
• dotnet run --project src/tooling/docs-migrate -- convert --majors 1 --minors 1 --book en/elasticsearch/reference
• grep -o 'file: .*' toc.yml | sort | uniq -d → empty (no duplicate entries)
• Sidebar nesting matches https://www.elastic.co/guide/en/elasticsearch/reference/8.19/index.html

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