Make Confluence REST API path configurable

[hkwi]

Context

Confluence Server installations can expose content resources below different REST API roots or require an explicit API version. This follows the URL compatibility issue described in atlassian-api/atlassian-python-api#1654.

Summary

• keep the current unversioned /rest/api/content/... path as the default
• allow the API root and version to be configured independently through environment variables or Git remote configuration
• apply the configured path to page reads, child traversal, space imports, attachment listings, and page updates
• keep attachment download and browser links site-relative instead of prefixing them with the REST API path
• document clone-time and configured-remote examples

For example, apiRoot=custom/api/root and apiVersion=2.0 produce /custom/api/root/2.0/content/....

Configuration

• CONFLUENCE_API_ROOT / GIT_REMOTE_CONFLUENCE_API_ROOT
• CONFLUENCE_API_VERSION / GIT_REMOTE_CONFLUENCE_API_VERSION
• remote.<name>.apiRoot / remote.<name>.apiVersion
• global fallback keys under confluence.* and remote.confluence.*

Testing

• go test ./...
• go vet ./...
• git diff --check

Tests cover defaults, environment variables, real Git remote configuration, custom content URLs, and site-relative attachment downloads.