Update and improve GitLab Functions documentation

Problem

Customers are curious about GitLab Functions (was CI Steps), and have expressed that the documentation is lacking in certain areas and is unclear.

Proposal

Investigate and/or update the following:

• Terms should be consistent
• Add a glossary to define GitLab Functions, a function, a step, inputs, outputs, enviroment
• Update the left menu to say Functions, not Steps
• Update the language in the tutorial https://docs.gitlab.com/tutorials/setup_steps/, and the tutorial itself, if necessary
• Update the install instructions for GitLab Administrators of Runner
• Make sure the examples work
• Review limitations and epics to make sure they're up-to-date
• Consider adding instructions on how to run a function locally
• Describe how to run functions on non-docker executors (particularly, the shell executor)
• Consider the readiness of OCI, and whether it should be documented
• Expect func.yml instead of step.yml
• Expect func: instead of step:
• Document Functions expressions (update job inputs docs to reference this page)
• Remove references the legacy step-runner image
• Deprecate fetching from git
• Rename CI Functions to GitLab Functions