[Repo Assist] docs: document --saveimages flag and image embedding patterns

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🤖 This PR was created by Repo Assist, an automated AI assistant.

Closes #683

Summary

This PR addresses the two documentation gaps reported in #683:

1. --saveimages flag was not explained — the command-line reference only said "Save images referenced in docs" with no mention of the none/some/all values.
2. No guidance on embedding image outputs — there was no documentation showing how to embed images (e.g. charts, plots) produced by evaluated scripts.

Changes

docs/commandline.md

• Expanded --saveimages table entry to document all three values (none/some/all) and which output formats each affects.
• Added a new Embedding Images section with:
  • A table summarising --saveimages behaviour.
  • An example of inline Base64 image embedding using include-it-raw (the pattern shown in the issue by @gbaydin).
  • A pointer to fsi.AddHtmlPrinter for custom type-based rendering.

docs/evaluation.fsx

• Expanded the "Using AddPrinter and AddHtmlPrinter" section (previously a single unexplained sentence) with:
  • An explanation of the fsi.AddHtmlPrinter API signature.
  • A concrete example showing how to register a printer that converts a chart value to an inline Base64 (img) tag.
  • An alternative example using a plain inlinePng helper with include-it-raw.

RELEASE_NOTES.md

• Added entry under [Unreleased] → Added.

Test Status

Documentation-only changes; no code modified. The existing test suite is unaffected.

Generated by Repo Assist for issue #683

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