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How can I create my own custom content-types that are representations of users, categories, articles etc?

Do not use a standard text/xml content-type. A client is not capable of handling this kind of information. Instead, use a custom format in the following form:

Content-type: application/vnd+company.category+xml
Content-type: application/vnd+company.category+html
Content-type: application/vnd+company.category+json

This allows your clients to process the information (in this case: your categories) as the specified content-type. All three of these content-types are categories, but they are represented in different formats (xml, html and json). Even though this is still information that clients need to know in advance, clients know what to expect, since they can ask for the specific information (categories, in this case), without outside knowledge like the URL etc.

All application/vnd are vendor-specific content-types and are not standardized. These are for non-standard content-types only.