Use env vars secret in deployment; update ci/cd; add readme

[jessebot]

Description

helps address small-hack/matrix-chart#556

Changes

• Actually use the secret we create as an environment variables for the deployment
• Change database name and usernames everywhere from matrix copypaste
• change default secret name to {{ include "matrix-sliding-sync.fullname" . }}-env
• Add root level README.md
• update ci/cd to point at matrix-sliding-sync-chart instead of matrix, and remove coturn
• update postgresql helpers templates and add networking policy

Outstanding tasks

• auto generate secret if not provided
• fix issue where db connection string isn't templating correctly
• update postgres parameter to match matrix chart in ci ref: switch from kind to k3s for testing in workflows and tweak postgresql persistence to get tests working again matrix-chart#532