Add build-docs file to add ddev build-docs command

[adiati98]

Description

This PR adds build-docs file to the commands/web folder to use ddev build-docs to build the HTML.

How to test

1. Pull this PR to your local.
2. Run ddev start.
3. Run ddev build-docs. It builds the docs.

As comparison:

1. In your terminal, navigate to the default 5.x branch.

2. Run ddev start.

3. Run ddev build-docs. You should see below error message:

   Error: unknown command "build-docs" for "ddev"
   Run 'ddev --help' for usage.

4. Run ddev exec make html. It builds the docs.

Linked issue

Closes #270

Screenshots or screen recordings