docs(examples): show reading CSV data from a file

[lunadogbot]

Summary

• add a file-reading variant to the CSV parsing example using Deno.readTextFile
• include a sample data.csv file in the example page
• update the local run command to show read permission

Fixes #2894.
Closes bartlomieju/orchid-inbox#110.

Verification

• deno fmt --check examples/scripts/parsing_serializing_csv.ts
• deno test -A examples/_pages/examples_test.ts
• deno task build:light
• confirmed _site/examples/parsing_serializing_csv/index.html includes Deno.readTextFile("./data.csv") and the data.csv snippet

[fibibot]

No issues found. The added Deno.readTextFile("./data.csv") sample is consistent with the new @run -R main.ts metadata, and deno test -A examples/_pages/examples_test.ts passes with the multi-file snippet.

[fibibot]

@bartlomieju this is ready to merge