docs: Update CLI page and add new environment variables page

[eric-schneider]

Summary

This PR does the following:

1. Updates the existing CLI page to match the current state in the latest Langflow release.
2. Adds a dedicated page about environment variables.

There are many pages/features in the docs that reference or directly involve using environment variables, but up until this point there has been no centralized page that catalogs all of them or describes their general usage. The goal of these updates is to provide make it easier to refer users to these pages for learning general information about the CLI and environment variable options rather than repeating the information in multiple locations across the docs.

Although the Markdown syntax isn't pretty, I was able to add anchors to every environment variable and option so that going forward, we can start linking to them directly from other pages.

Need reviewer help

1. All environment variables and CLI options are documented in tables. The markdown syntax is difficult to parse, so I highly recommend building the docs and reviewing the HTML.
2. I used the CLI options and .env.example as a guide for which environment variables to include, but I need feedback on whether there are any variables missing (or if there are any variables that should be removed).
3. I tried to scrape as much information as I could from the code to determine default values and descriptions, but some info is still missing (and all of the info needs to be verified by SMEs because I'm not fully confident that everything is accurate).