Document in Vuepress how to add a custom logo

[derick-montague]

Overview

Document in the theme section of the Vuepress documentation the:

1. Image naming conventions
2. Expected image type
3. Guidance about the Meta Data layer used to store the custom image
4. Adding an org specific .env file and the values required to support logos

[derick-montague]

Waiting for actual logo work to be completed in the Gerrit review prior to adding documentation.

https://gerrit.openbmc-project.xyz/c/openbmc/webui-vue/+/34094