Readme cleanup, add dev repo & FAQ, fix formatting, bump to 1.7.2

[happyloa]

Motivation

• Remove an invalid external issue link and outdated roadmap note from the public readme.txt so the published plugin listing is accurate.
• Fix the cron example formatting so code renders consistently with other CLI Usage examples.
• Provide clear admin guidance for re-sending failed notification payloads from the Watchdog UI.
• Align on-disk metadata with the released plugin version.

Description

• Removed the _Future enhancement_ roadmap sentence and the broken GitHub issue link from readme.txt and added a 1.7.2 changelog entry.
• Replaced the fenced code block with an inline code example for the cron endpoint (curl -X POST https://example.com/wp-json/site-add-on-watchdog/v1/cron) and added an FAQ entry explaining how to resend failed notification payloads.
• Added a Development section linking to https://github.com/happyloa/Site-Add-on-Watchdog and noted the addition in the 1.7.2 changelog.
• Bumped version metadata to 1.7.2 by updating Stable tag in readme.txt, the plugin header Version in site-add-on-watchdog.php, and Version::NUMBER in src/Version.php.

Testing

• No automated test suite was executed because the changes are documentation and constant updates only.
• Changes were staged and committed successfully across multiple commits (readme edits, formatting fixes, and version bumps).
• Verified the affected files were updated: readme.txt, site-add-on-watchdog.php, and src/Version.php.
• Packaging/CI and runtime tests were not invoked as part of this change.

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Codex Task