add page-chatbot attribute to enable chatbot

[adam-cowley]

The page-chatbot attribute sets the base URL of the folder containing the HTML and CSS built by the docs-chatbot repo.

:page-chatbot: https://neo4j.com/static/chatbot/

A PR to sync the js/css assets has been opened at: https://github.com/neo4j/docs-chatbot/pull/8

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[recrwplay]

To avoid accidentally making the chatbot available anywhere we don't want to yet, this attribute should not be set directly in a docs project, either by adding to a playbook or component descriptor (antora.yml).

The attribute can be added as part of a CI build by using the Antora CLI - https://docs.antora.org/antora/latest/cli/options/#name-value - and this is how it is currently being added to the staging server docs.