Add .pkgdownignore support (#2959)

[kyleGrealis]

Summary

This PR adds support for a .pkgdownignore file that allows users to exclude specific markdown files from being rendered to HTML during build_home().

Fixes #2959

Motivation

With the rise of AI coding assistants, developers are increasingly adding files like CLAUDE.md, AGENTS.md, and COPILOT.md to their package roots. These files are developer-facing and shouldn't appear in user documentation or site search results.
Currently, there's no way to exclude arbitrary markdown files from pkgdown builds. The only options are:

• Move files elsewhere (breaks tool conventions)
• Delete before CI builds (fragile)
• Prefix with _ (only works for vignettes)

Solution

A simple .pkgdownignore file that lists filenames to exclude:

# AI assistant configuration
CLAUDE.md
AGENTS.md

# Internal docs
INTERNAL.md
Features

• Supports comments (#) and blank lines
• Can be placed in package root, pkgdown/, or _pkgdown/
• Files from all locations are combined and deduplicated
• Matches by basename (works for both root and .github/ files)
• Displays ignored files during build: Ignoring: CLAUDE.md and AGENTS.md

Changes

• R/build-home-md.R: Added read_pkgdownignore() helper and integrated into package_mds()
• tests/testthat/test-pkgdownignore.R: 15 tests covering all functionality
• NEWS.md: Added bullet for this feature

Example

# .pkgdownignore
CLAUDE.md
AGENTS.md

In the build output:

── Building home ───────────────────────────────────────────────────────
Ignoring: CLAUDE.md and AGENTS.md
Reading .github/CODE_OF_CONDUCT.md
Reading .github/CONTRIBUTING.md