Fix LD_LIBRARY_PATH and documentation images with transparency

[SuhasSrinivasan]

This PR contains two main changes

1. Update LD_LIBRARY_PATH export format for compatibility.
   As discussed in Documentation for libtorch installation #491
2. Having a white background makes it easy to view the image in Dark Mode (OS/Browser/Site) and for Visual Accessibility.
   Fix for issue Black lines and black text are not visible in Open Chromatin Illustration #572
   2.a. Open Chromatin illustration
   2.b. Methylation Entropy illustration

Closing the earlier separate PRs #569 and #573 since current PR encapsulates all changes.

[Copilot]

Pull request overview

This PR updates runtime setup notes for the tch/libtorch build and adjusts documentation images to improve visibility in dark mode / non-white backgrounds.

Changes:

• Update LD_LIBRARY_PATH export logic to preserve an existing value while appending the libtorch library directory.
• Switch two docs pages to reference “no alpha” (non-transparent) versions of illustrations.
• Add a new “no alpha” entropy illustration image.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 2 comments.

File	Description
docs/intro_open_chromatin.html	Switches the open chromatin illustration to a non-transparent PNG.
docs/intro_entropy.html	Switches the methylation entropy illustration to a non-transparent PNG.
docs/images/me_no_alpha.png	Adds a non-transparent version of the methylation entropy illustration.
BUILD_NOTES_tch.txt	Makes LD_LIBRARY_PATH export append-safe for compatibility with existing user environments.

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