Add Imaging Modalities section to tutorial index

[muratmaga]

Reorganizes the top-level tutorial index in README.md.

• New Imaging Modalities section, placed first under "Tutorial descriptions", collecting the three background tutorials moved verbatim out of Others: What is a microCT, What is photogrammetry, What is surface 3D scanning. They describe how the data is acquired, so they read better ahead of the import tutorials.
• Removed three outdated Discourse thread links from Others (reorient/resample, multi-channel RGB microscopy rendering, sequences for rendering states).

Documentation only — no tutorial content changed, no files moved on disk.

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[claude]

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Review: Add Imaging Modalities section to tutorial index

• Gather context (diff, CLAUDE.md, README structure)
• Review changes for correctness/regressions
• Post final review

Summary: This is a documentation-only reorganization of README.md and I found no correctness bugs, security issues, or regressions.

Verified:

• The three moved entries (What is a microCT, What is photogrammetry, What is surface 3D scanning) now live under the new ### Imaging Modalities section (README.md:6-9) and are not duplicated elsewhere.
• Their target files (microCT/README.md, photogrammetry/README.md, surfaceScanning/README.md) exist, so the links resolve correctly.
• The three removed Discourse links are gone from ### Others (README.md:91-94) with no dangling references left behind.
• No repo has a CLAUDE.md with conventions this change would need to follow.

Minor, non-blocking observation (pre-existing, not introduced by this PR): the numbered lists throughout README.md (e.g. ### Data Import/Export..., ### Geometric Morphometrics...) have long had out-of-sequence numbering (1, 3, 4, 5, 7, 2, 3...). The new "Imaging Modalities" section itself is numbered correctly (1, 2, 3), so this PR doesn't add to the problem — just flagging for awareness, no action needed here.

Since this PR is already merged, no further changes are proposed.