docs(extraction): pin GitHub source links to the 26.08 tag (NVBug 6628516)

[kheiss-uwzoo]

Summary

• Pin release-specific blob/main and tree/main GitHub links in versioned 26.08 extraction docs to the immutable 26.08 tag (Helm README/values, CLI, examples, source, nav, and redirects).
• Leave contribution-policy links on main (contributing.md → CONTRIBUTING.md).
• Fixes NVBug 6628516: after main advances, 26.08 readers no longer follow newer Helm keys, defaults, examples, or source.

Why this is on main

There is no 26.08 maintenance branch. Tag 26.08 currently matches upstream/main (5d9a42a57). This pin is for the 26.08 versioned docs snapshot.

After main diverges for the next train, restore /blob/main/ and /tree/main/ on main (or keep this pin only on a 26.08 branch) so latest docs stay evergreen.

Intentional leftovers

• docs/docs/extraction/contributing.md still points at blob/main/CONTRIBUTING.md (evergreen, per bug expected result).
• docs/docs/license.md still has four blob/main links (outside the extraction audit scope).
• In-repo Helm/CLI markdown still contains blob/main self-links; this PR only changes published extraction docs and docs/mkdocs.yml.

Test plan

• git grep for github.com/NVIDIA/NeMo-Retriever/(blob|tree)/main under docs/docs/extraction — only contributing.md remains
• Sample 26.08 tag URLs resolve (Helm README exists; heading IDs such as #secrets, #changing-nim-image-repository-or-tag, #recommended-minimal-install-2608 are present)
• python -m mkdocs build --strict --config-file mkdocs.yml from docs/ — PASS when untracked leftover pages are not in the tree
• Click Helm README, values.yaml, CLI, and example links from 26.08 release notes and deployment docs after publish
• Confirm contribution links still open CONTRIBUTING.md on main

pre-draft: leakage, mkdocs --strict, ::a, ::p, ::r on the diff vs main

Base: upstream/main
Files: agentic-retrieval-concept.md, api-keys.md, audio-video.md, concepts.md, content-metadata.md, customize-extend.md, deployment-options.md, embedding.md, faq.md, getting-started-about.md, multimodal-extraction.md, nemo-retriever-api-reference.md, overview.md, performance_guide.md, prerequisites-support-matrix.md, releasenotes.md, troubleshoot.md, vdbs.md, workflow-agentic-retrieval.md, workflow-document-ingestion.md, workflow-e2e-blueprints.md, mkdocs.yml

Check	Result
Leakage (page roles + see [ CTAs)	PASS — no see [ CTAs on changed files; leakage script FAIL was untracked leftover custom-metadata.md, not in this diff
Allowed paths	PASS — 22 documentation files (docs/docs/extraction/**/*.md + docs/mkdocs.yml)
mkdocs --strict	PASS — exit 0 after moving untracked leftover pages aside; those leftovers are not in this diff
::a audit	PASS — URL retarget only; 26.08 tag exists and Helm heading IDs used by these links are present (95%)
::p polish	none needed — no prose rewrite; GitHub refs only
::r style	100% — no new sentences; no blocking issues

Code drift (not in this docs PR): nemo_retriever/helm/README.md and other in-repo guides still contain blob/main self-links.

PR: this draft