Move sparklines guide to mdx

[alexeyraspopov]

I'll try to slowly move all existing guides to MDX, starting from the pages that are currently broken on semiotic.nteract.io. Sparklines is the first one.

There is a small detail about this particular guide that we need to keep in mind. In MDX version of it, using JSX elements right in the text (to make use of sparklines) may sometimes add a line break because of Prettier. The only way to avoid it (other than trying to rephrase the guide) is to put   between the inline element and the word before it. The root cause is the fact I use prose wrap which helps keeping some readable print width when working with text.

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