Include a note on how to configure the list of allowed APIM documents types

[msm1992]

Description:
Below configuration in APIM supports configuring the allowed file types as API documents. The default set of file types allowed are "pdf", "txt", "doc", "docx", "xls", "xlsx", "odt", "ods", "json", "yaml" and "md".

[apim.publisher]
supported_document_types = "pdf, txt, doc"

Please add this info as a note to doc[1]. This config is applicable to all APIM versions above 3.2.0.

[1]. https://apim.docs.wso2.com/en/3.2.0/learn/design-api/api-documentation/add-api-documentation/#add-documentation-using-a-file

Affected Product Version:
3.2.0 onwards

Related Issues:
wso2/product-apim#11500

[vishmi49]

Documentation is updated with following PRs

[3.2.0] #8302
[4.0.0]#8309
[4.1.0] #8310
[4.2.0] #8311
[4.3.0]#8312
[master]#8313