Build Dash docset version of the spec

[Dash](https://kapeli.com/dash) and [Zeal](https://zealdocs.org/) are popular offline documentation applications that use the Dash docsets.

It would be neat to have the Matrix specifications available on these applications for offline usage. Instructions to do so can be found here: https://kapeli.com/docsets

Read the Dash docsets instructions linked in the issue and inspect the repository's HTML specification output. Done means the Matrix specifications are packaged as a Dash docset and can be used offline in Dash or Zeal.

Written by the indexing model from the issue text.