[Docs] add mortonEncode and mortonDecode

[Blargian]

Closes #1984 as part of the project to document missing or incomplete functions in the documentation.

• Added mortonEncode and mortonDecode to the docs (documentation was already written in the .cpp file, just needed to be replicated with some format tweaks).

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• Documentation (changelog entry is not required)

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[Blargian]

@johnnymatthews could you give this a quick lookover when you are able? Do we need to add examples using columns as arguments? There is an example like this (see below) in the function source file however as I understood functions in the documentation should be able to be run by copy pasting into ClickHouse play online which is why I excluded it and opted to just mention this possibility in the 'parameters' section.

SELECT mortonEncode((1,2), n1, n2) FROM table

[robot-clickhouse]

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[nikitamikhaylov]

@Blargian You can just write some complex query using a subquery to emulate that you are selecting from a table.

[Blargian]

@nikitamikhaylov thanks, I will add it