BENCH: programmatically create benchmarks for large ngroups (GH6787)
closes #6787

Uses ngroups=10000 as suggested in the issue, which takes about 1 hour on my desktop.

For results (vb_suite.log, pkl file) see: https://gist.github.com/dlovell/ea3400273314e7612f6e
Note: gist references a different commit hash.  I changed the commit message and added modification to doc/source/v0.15.0.txt, but the actual modifications to vb_suite/groupby.py are principally the same.

@dlovell  this is gr8!

can u add small but for small groups as well? (say 100?
and name very similarly)

might then have some duplicates (well some that do very similar - so can eliminate those and just use your new ones)

pls also create an issue to investigate the perf of some of these (just paste you test run in  new issue for reference) 
 Commit c494c03 runs the tests for ngroups values 100 and 10000.  Output looks like

```
groupby_ngroups_100_describe                 | 780.9353 |
groupby_ngroups_10_cumcount                  |   0.3180 |
```
 @dlovell thanks!
