docs: add missing v1.2.21 features to documentation (#23085)

## Summary
- Added documentation for 5 features introduced in Bun v1.2.21 that were
missing from the docs
- Kept updates minimal with high information density as requested

## Changes
- **bun audit filtering options** (`docs/install/audit.md`)
  - `--audit-level=<low|moderate|high|critical>` - filter by severity
  - `--prod` - audit only production dependencies  
  - `--ignore <CVE>` - ignore specific vulnerabilities

- **--compile-exec-argv flag** (`docs/bundler/executables.md`)
  - Embed runtime arguments in compiled executables
  - Arguments available via `process.execArgv`

- **bunx --package/-p flag** (`docs/cli/bunx.md`)
  - Run binaries from specific packages when name differs

- **package.json sideEffects glob patterns** (`docs/bundler/index.md`)
  - Support for `*`, `?`, `**`, `[]`, `{}` patterns

- **--user-agent CLI flag** (`docs/cli/run.md`)
  - Customize User-Agent header for all fetch() requests

## Test plan
- [x] Reviewed all changes match Bun v1.2.21 blog post features
- [x] Verified documentation style is concise with code examples
- [x] Checked no existing documentation was removed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

docs/bundler/executables.md

@@ -140,6 +140,19 @@ The `--sourcemap` argument embeds a sourcemap compressed with zstd, so that erro
 
 The `--bytecode` argument enables bytecode compilation. Every time you run JavaScript code in Bun, JavaScriptCore (the engine) will compile your source code into bytecode. We can move this parsing work from runtime to bundle time, saving you startup time.
 
+## Embedding runtime arguments
+
+**`--compile-exec-argv="args"`** - Embed runtime arguments that are available via `process.execArgv`:
+
+```bash
+bun build --compile --compile-exec-argv="--smol --user-agent=MyBot" ./app.ts --outfile myapp
+```
+
+```js
+// In the compiled app
+console.log(process.execArgv); // ["--smol", "--user-agent=MyBot"]
+```
+
 ## Act as the Bun CLI
 
 {% note %}