simplify setup for tenant specific docs

[tronghn]

This merges in the tenants directory into the regular docs directory, which means that files and directories specific to a tenant now lives together with the rest of the common content.

To mark a page as specific for a given tenant, add the conditional tag to the markdown front matter:

---
conditional: [tenant, <tenant name>]
---

To build locally for a specific tenant, use the tenant argument when invoking make local:

make local tenant=<tenant name>

Caveats:

• This is made possible by using the https://github.com/DariuszPorowski/mkdocs-file-filter-plugin plugin. The plugin doesn't work with mkdocs version 1.6.x as of now, so we'll have to be aware of that when upgrading.

[mortenlj]

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