doc: Adds Changelog for v1.15.2

[maastha]

Description

This PR adds Changelog for v1.15.2 release.

Link to any related issue(s): https://jira.mongodb.org/browse/CLOUDP-237981

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[andreaangiolillo]

Should we also update the link to the example in the doc main page to point to the new tag?

[maastha]

not sure if we reached a consensus on that, @AgustinBettati could you confirm?

[maastha]

discussed offline, we'll update the link and outline the procedure during release with a separate ticket for future releases

[jwilliams-mongo]

LGTM