mark cli_alert*() functions as superseded

[simonpcouch]

Working on a cli cheatsheet with a tidy dev day participant and we were trying to figure out how cli_alert*() functions should be situated in the cheatsheet. It was my sense that they are essentially a different way of wrapping cli__message() that automatically prepend the x and ! icons, and their use case wasn't clear to me—Hadley confirmed that those functions are now superseded in favor of e.g. cli_inform() with named elements.

The cli_alert*() functions likely ought to be marked as superseded in the documentation and removed (or pushed later) from the first section of "Building a semantic cli."

[cynthiahqy]

If superseded, these lines/content should be updated:

Move into Superseded section of Reference Index:

cli/_pkgdown.yml

Lines 64 to 68 in 2ddcc1a

	- cli_alert
	- cli_alert_danger
	- cli_alert_info
	- cli_alert_success
	- cli_alert_warning

Remove from Building a Semantic CLI vignette

cli/vignettes/semantic-cli.Rmd

Lines 59 to 82 in 2ddcc1a

	## Alerts
	Alerts are typically short messages.
	cli has four types of alerts (success, info, warning, danger) and also a generic alert type:
	```{asciicast}
	cli_alert_success("Updated database.")
	```
	```{asciicast}
	cli_alert_info("Reopened database.")
	```
	```{asciicast}
	cli_alert_warning("Cannot reach GitHub, using local database cache.")
	```
	```{asciicast}
	cli_alert_danger("Failed to connect to database.")
	```
	```{asciicast}
	cli_alert("A generic alert")
	```

[cynthiahqy]

cli_alert_*() also appears in the usethis_ui vignette -- e.g. https://cli.r-lib.org/articles/usethis-ui.html#with-cli-8