Add filter documentation

##### SUMMARY

ansible-core 2.14 adds 'side car docs' which gives collection owners the ability to add documentation for previously undocumentable filter and test plugins.

This requires creating yaml files in the same directory as the filter or test plugin code, one yaml file per plugin.

See https://github.com/ansible/ansible/blob/devel/lib/ansible/plugins/filter/b64decode.yml for an example.

The following two filters need this documentation:
[gcp_kms_decrypt filter](https://docs.ansible.com/ansible/devel/collections/google/cloud/gcp_kms_decrypt_filter.html#ansible-collections-google-cloud-gcp-kms-decrypt-filter) –

[gcp_kms_encrypt filter](https://docs.ansible.com/ansible/devel/collections/google/cloud/gcp_kms_encrypt_filter.html#ansible-collections-google-cloud-gcp-kms-encrypt-filter) –

Related to https://github.com/ansible-community/community-team/issues/65

##### ISSUE TYPE
- Documentation Report

##### COMPONENT NAME

##### ANSIBLE VERSION

```paste below

```

Start with the linked gcp_kms_decrypt and gcp_kms_encrypt filter documentation pages, then review the ansible-core b64decode.yml example to understand the side-car format. Create documentation for both filters in their collection plugin locations, and verify that each filter has the expected generated documentation.

Written by the indexing model from the issue text.