docs: fix the docs/README.md index intro and entry separator inconsistency

[inureyes]

Two small consistency problems in docs/README.md: the intro says "both" before a three-item list, and two of the nineteen document-index entries use a different title separator than the other seventeen.

Evidence

docs/README.md:4:

It may contain both:

followed immediately by a numbered list of three item kinds (GitHub-facing Markdown documents, MkDocs site content, Git/GitHub workflow documents). "Both" cannot introduce three items.

In the "Current GitHub-facing docs" index in the same file, entries 13 and 19 use a colon after the file name:

13. `python-client.md`: the `mlxcel` Python client over the OpenAI-compatible server, ...
19. `code-guidelines.md`: the file-size and module-split thresholds, ...

while the other 17 entries (docs/README.md:17-33) separate the file name from the description with an em dash, for example entry 1 (installation.md) and entry 12 (block-diffusion.md).

Suggested fix

• Change docs/README.md:4 to "It may contain:" or "It may contain any of three kinds:".
• Rewrite entries 13 and 19 to use the same em dash separator the other 17 entries use.

Acceptance criteria

• The intro at docs/README.md:4 is grammatical for a three-item list
• All 19 index entries use one separator style (the em dash style of the current majority)