docs: Document install procedure for Azure Functions for Linux

[mrickard]

Description

For Windows, the Node agent can be installed via the New Relic Azure Site Extension. For Linux, customers can install via one of these methods:

• installing as a dependency in package.json
• injecting the agent in a Docker file for containerized functions
• possibly using an init container for containerized functions (tested with web apps; not with functions)
• and other methods as they become available

There's a confluence document on these procedures for Web Apps; we should verify that they behave as expected for Functions Apps.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-400769

[mrickard]

The most common installation scenarios (direct install, direct install using containers) are documented. Init container for functions require particularly detailed permissions and resources, so are less likely to be used initially.