Internal: build documentation refers to obsoleted Jenkins instead of Gradle

[mi-hol]

File found with incorrect information is:

/.github/workflows/readme.md

[nmschulte]

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams#creating-mermaid-diagrams

https://mermaid.js.org/syntax/flowchart.html

flowchart TD
    gha_firmware["Firmware at GHA"]
    firmware_compile[["firmware/compile.sh"]]
    etc{{"..."}}
    gha_firmware --> firmware_compile
    firmware_compile --> etc