Function Explanation: check_availability

The check_availability function performs the following tasks:

1. Writes the header for the report, including the current date and time, to the report file.
2. Iterates over each URL in the URLS array.
3. For each URL, it uses curl to measure the HTTP status code. The curl command is used with the following options:
   • -o /dev/null: Discards the output.
   • -s: Silent mode, which suppresses progress and error messages.
   • -w "%{http_code}\n": Writes the HTTP status code to the variable.
   • -L: Follows redirects.
4. Checks if the HTTP status code is 200. If it is, it appends a message stating the URL is available to the report file. Otherwise, it appends a message stating the URL is not available, along with the HTTP status code.
5. Prints a message indicating that the report has been saved.