Update subs documentation

[theletterf]

• The substitutions.md page mistakenly suggests subs:, when according to the code it's sub:
• The frontmatter.md docs is lacking information on the sub: field

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/frontmatter.md
• docs/syntax/substitutions.md

[leemthompo]

Unrelated to your changes, but while here I'd add a link to https://github.com/elastic/docs-builder/blob/main/config/products.yml under Substitutions

i.e.:

Product substitutions use products.yml to determine what will be displayed. Use the product's key to get its display name.