Add tutorial steps

[jzohrab]

The vextab tutorial (https://www.vexflow.com/vextab/tutorial.html) doesn't yet mention that vextab supports multiple voices or fingerings, though there are tests showing that it's possible. This PR is a documentation-only update. I tried to match the style and tone of the prior sections, but please edit them as you see fit.

I did note that the TAB section doesn't render correctly when the score is complicated, is that why this hasn't been documented yet?

Cheers and regards. Vexflow and Vextab are incredible pieces of work! jz

[jzohrab]

ps - I tried various ways of writing a "system of staves" for each voice, but they never came out properly connected. The below just creates separate staves, and nothing lines up:

tabstave notation=true
voice
tabstave notation=true
notes :8 6/4 h7/4 :q 0/2 :8 T0/2 5/3 :q 3/2 | :q T3/2    :16 3/2 0/1 :8 6/2 :q T6/2 :q 0/1 |
voice
tabstave notation=true
notes :w ##                                 | :8 ##  5/3 :8  0/2        7/4 6/4 7/4 :q 0/2 |
voice
tabstave notation=true
notes :q ## :8 0/4 7/5 :qd 3/4 :8 0/4       | :q 2/4 |
voice
tabstave notation=true
notes :q 7/6 7/6 0/5 0/5                    | 4/6 0/5 7/6 4/6 |

Let me know if there's a way to do this ... I couldn't figure it out from the tests or the jison. Thank you! jz

[jzohrab]

I see that the tab/score bar misalignment is discussed in #99.

[jzohrab]

With #122 the barline issue is fixed.