chore: add link to PyPI history in CHANGELOG.md

[parthea]

Fixes googleapis/librarian#2246

[PyPI History][1]

[1]: https://pypi.org/project/google-ads-admanager/#history

The anchor pattern below needs to be in each CHANGELOG.md to identify the top of a CHANGELOG file as per the code below:

google-cloud-python/.generator/cli.py

Lines 766 to 767 in 692a39c

	"""This function searches the given content for the anchor pattern
	`[1]: https://pypi.org/project/{library_id}/#history`