Added a paragraph about the pygeoapi HTML encoding on the template section of the documentation

[doublebyte1]

Overview

There are often requests to add more functionality to the UI of pygeoapi. IMHO pygeoapi is an API, and thus it was designed to interact with machines, mainly with JSON or another machine readable encoding. The frontend is provided only as an example of a HTML encoding, and by design, it has limited functionality.

This PR adds a paragraph to clarify this in the documentation.

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines