docs: add cnd() to pkgdown reference index

[LeonidasZhak]

Fixes #1900

Problem

The cnd(), error_cnd(), warning_cnd(), and message_cnd() constructors are documented in man/cnd.Rd and exported in NAMESPACE, but were missing from the pkgdown reference index at https://rlang.r-lib.org/reference/index.html.

Fix

Added cnd to the "Conditions" subsection in _pkgdown.yml.

Validation

• YAML syntax validated with yaml::yaml.load_file()
• The cnd entry resolves to man/cnd.Rd which documents all four constructors
• cnd_muffle and cnd_type are intentionally omitted as they are marked \keyword{internal}

Diff

Single line addition in _pkgdown.yml:

- subtitle: Conditions
  contents:
+ - cnd
  - rlang_error
  - cnd_message
  - format_error_bullets
  - cnd_inherits

[lionel-]

Thank you