doc: document -blocknotify, -walletnotify and other -*notify options #35518

Partially addresses #14278, which asks for documentation of the user-facing -*notify behaviors. The five shell-command notification options currently have no documentation outside of --help text, and several behaviors are only discoverable by reading the source.

Adds doc/notifications.md (indexed in doc/README.md) covering:

-blocknotify

• Suppressed during initial block download and reindex — only fires once the node reaches the post-init sync state (src/init.cpp, NotifyBlockTip_connect handler)
• Each event runs the command in a detached thread, so there is no ordering guarantee between successive notifications — this directly documents the behavior behind the ordering question raised in #14278 and the test discrepancy noted in #14275

-walletnotify

• Fires on mempool entry, confirmation, and when a transaction returns to unconfirmed due to a conflict with a newly connected block (CWallet::transactionRemovedFromMempool)
• Documents all four substitutions (%s, %b, %h, %w) and the Windows limitation for %w

-alertnotify

• Fires once per distinct warning condition; message is sanitized and single-quoted before substitution (src/node/kernel_notifications.cpp)

-startupnotify / -shutdownnotify

• Startup command does not block initialization; shutdown commands may be specified multiple times, run in parallel, and shutdown waits for them to complete (src/init.cpp)

All described behavior is derived directly from the source.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35518.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->