[gh-pages] add 'reports' sub-section with initial performance measurements

[lukaszstolarczuk]

Preview: https://lukaszstolarczuk.github.io/pmemkv/#performance-reports

// the external storage space is just too long waiting time, so for now we publish this one into GH; later on, we move on to the new storage place (I hope 😸 )

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This change is 

[karczex]

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reports/index.md, line 1 at r2 (raw file):

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Is reports directory really needed? Probably you may just add perdormace_reports.md to main directory. Or even add section to README.md.