Fix links in templates, gcode.h (#14160)

.github/issue_template.md

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+Have you read Marlin's Code of Conduct? By filing an Issue, you are expected to comply with it, including treating everyone with respect: https://github.com/MarlinFirmware/Marlin/blob/bugfix-2.0.x/.github/code_of_conduct.md
 
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Marlin/src/gcode/gcode.h

  * -----------------
  *
  * Helpful G-code references:
- *  - http://linuxcnc.org/handbook/gcode/g-code.html
- *  - http://objects.reprap.org/wiki/Mendel_User_Manual:_RepRapGCodes
+ *  - http://marlinfw.org/meta/gcode
+ *  - https://reprap.org/wiki/G-code
+ *  - http://linuxcnc.org/docs/html/gcode.html
  *
  * Help to document Marlin's G-codes online:
- *  - http://reprap.org/wiki/G-code
  *  - https://github.com/MarlinFirmware/MarlinDocumentation
  *
  * -----------------