MI - SECURITY section (Symmetric Encryption doc)

[nilminiwso2]

The content on symmetric encryption [1] rewritten for [2].
Please see preview on netlify[3].

[1] https://docs.wso2.com/display/ADMIN44x/Using+Symmetric+Encryption
[2] https://github.com/wso2/docs-ei/blob/master/en/micro-integrator/docs/setup/security/single_key_encryption.md

[3] https://wso2docs-configurationcatalog-3.netlify.com/setup/security/single_key_encryption/

[nilminiwso2]

Completed the doc [1] using toml configurations.
The old content is as follows[2].
Preview on netlify [3]

[1] https://github.com/wso2/docs-ei/blob/master/en/micro-integrator/docs/setup/security/single_key_encryption.md
[2] https://docs.wso2.com/display/ADMIN44x/Using+Symmetric+Encryption
[3] https://wso2docs-configurationcatalog-3.netlify.com/setup/security/single_key_encryption/

[nilminiwso2]

@madushadhanushka - Could you please review?
Thanks!

[nilminiwso2]

https://ei.docs.wso2.com/en/latest/micro-integrator/setup/security/single_key_encryption/