PR for automatic paired functions cross-referencing

[avoroshilov]

Originally defined in issue #608
Short description:

1. Two config files added: doc/specs/vulkan/config/vulkan-api-tuples.txt and doc/specs/vulkan/config/vulkan-api-sorting.txt.
2. vulkan-api-tuples.txt defines paired functions, pairs delimiter is empty string.
3. vulkan-api-sorting.txt defines the order how prefixes appear in the cross-references list, * (meaning "any other function) denotes place where functions that didn't match any prefix go; function keep case-sensitive alphabetical order within the prefix, as it was prior to this change.
4. doc/specs/vulkan/genRef.py is modified to read, process and apply data supplied by the two config files mentioned.
5. doc/specs/vulkan/Makefile modified to have the config files as dependencies, to facilitate rebuild when config files changed
6. doc/specs/vulkan/chapters/pipelines.txt modified to have explicit cross references, due to the way paired functions are named: (vkCreateComputePipelines, vkCreateGraphicsPipelines) vs vkDestroyPipeline.

The idea behind this way of implementing the cross-references is in this comment.

There is a problem with pairing functions, provided by extensions (e.g. vkCreateSwapchainKHR vs vkDestroySwapchainKHR) - they are NOT automatically generated. Manual cross-referencing won't work too, because doc/specs/vulkan/vkapi.py simply doesn't have anything but the core functions, hence auto-cross-reference validation will fail and functions won't appear on the reference list. Manual referencing via adding xref - won't work either, because macroPrefix function in doc/specs/vulkan/genRef.py will not recognize the reference properly and return "UNKNOWN" prefix. Hence, in this revision paired extension functions do not get cross-references.

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