Add ANSI color rendering to validation output code blocks

[yarikoptic]

Summary

The validation documentation code blocks (e.g., $ dandi validate ... examples) currently (PR yet to be sent for extended docs on validate, TODO) show plain uncolored text. dandi validate uses Click's secho to color output by severity (red=ERROR, yellow=WARNING, blue=HINT), but the generation script strips ANSI codes since standard markdown code blocks can't render them.

Proposed Solution

Use pygments-ansi-color — a Pygments lexer plugin that parses ANSI escape sequences and renders them as colored HTML. It registers an ansi-color language identifier that works with pymdownx.highlight.

Implementation Steps

1. Add dependency: pygments-ansi-color to requirements.txt (requires Python >= 3.10)

2. Add CSS: Either:

   • Use markdown-exec[ansi] which bundles pygments-ansi-color + ready-made CSS with light/dark theme support
   • Or manually add an ansi.css file (e.g., from markdown-exec's assets) to docs/css/ and reference it in extra_css

3. Update generation script (scripts/generate-validation-examples.sh):

   • Force color output: set FORCE_COLOR=1 or equivalent env var for dandi-cli/Click
   • Do not strip ANSI codes from the output files that will use ansi-color blocks
   • Keep sanitizing absolute paths

4. Update code blocks in docs/user-guide-sharing/validating-files.md:

   • Change ```console to ```ansi-color for blocks that show dandi validate output
   • The ANSI escape sequences in the example files will be rendered as colored HTML

Example

A code block like:

```ansi-color
[31m[ERROR][0m [pynwb.GENERIC] sub-RAT123/sub-RAT123.nwb — nwb_version '2.0b' is not a proper semantic version.
[33m[WARNING][0m [NWBI.check_subject_age] sub-RAT123/sub-RAT123.nwb — Subject age, '12 mo', does not follow ISO 8601...
[34m[HINT][0m [BIDS.README_FILE_MISSING] dataset_description.json — The recommended file /README is missing.
```

Would render with red/yellow/blue colors matching what users see in their terminal.

Caveats

• The example files must contain literal ANSI escape bytes, not the text representation \033[31m
• pygments-ansi-color supports standard 8 colors + bright variants + bold — sufficient for Click's secho output
• Need to verify colors render well against both mkdocs-material default and slate (dark) themes
• Also consider click-extra which extends pygments-ansi-color with ANSI-aware shell session lexers (could allow ```console blocks with colored output)

Related

• https://github.com/chriskuehl/pygments-ansi-color
• https://github.com/pawamoy/markdown-exec (bundles ansi support)
• https://kdeldycke.github.io/click-extra/pygments.html (ANSI-aware console lexer)