Generate parts of the documentation

[GuptaManan100]

Description

We should generate parts of the docs which can be generated from the code in vitess.io website. For example, we have a list of all the reserved connections we support, that entire list is just a copy of list of variables from sysvars.go file.

[ajm188]

See #1318 for a possible way to go about doing this (that does the generation of vtctldclient docs for multiple versions at once). We can extract some of the common stuff (managing vitess dir's, git operations, etc) to some shared util package as well).

[GuptaManan100]

The links to specific files and line numbers should also be generated, so that if the files change, the links still point to the correct line.