docs: Add Binary Type

[MichelDiz]

Doc Binary Type.

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This change is 

[danielmai]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @MichelDiz)

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query/common_test.go, line 255 at r1 (raw file):

created_at                     : datetime @index(hour) .
updated_at                     : datetime @index(year) .
bin_data                         : binary .

nit: align colons like the rest of the schema.

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query/common_test.go, line 437 at r1 (raw file):

		<101> <alias> "John Oliver" .

		<1> <bin_data> "YmluLWRhdGE="^^<xs:base64Binary> .

This is nice to be explicit, even though it's not needed when the schema's already set to bin_data: binary . 👍

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wiki/content/mutations/index.md, line 207 at r1 (raw file):

See the section on [RDF schema types]({{< relref "query-language/index.md#rdf-types" >}}) to understand how RDF types affect mutations and storage.

Thanks for fixing the link!

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wiki/content/query-language/index.md, line 2010 at r1 (raw file):

|  `string`   | string  |
|  `bool`     | bool    |
|  `binary`   | base64  |

I think binary is a []byte in Go according to conversion.go.

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wiki/content/query-language/index.md, line 2030 at r1 (raw file):

#### Binary Type

The `binary` represents arbitrary Base64-encoded binary data. For base64Binary data the entire binary stream is encoded using the Base64 Encoding.

Need to double-check that the data is stored as a []byte but returned in JSON responses and exports as base64-encoded strings.