Adds principal engineer role to development department

Why is this change being made?

With the work done by the IC gearing ratio working group, the Principal Engineer role has been created at GitLab. In an effort to reflect the organisation structure already existing for job families and job roles at GitLab, this MR is duplicating the Principal Engineer role from /job-families/engineering/principal-engineer/ into /job-families/engineering/Development/principal-engineer/ to enable the ~"Development Department" to better reflect its needs without impacting the needs of other sub-departments within Engineering.

What are the changes?

• Create sub directory for Principal Engineer under /job-families/engineering/development/
• Duplicate /job-families/engineering/principal-engineer/index.html.md within /job-families/engineering/development/

Consider?

• Update Development Department career development framework: https://about.gitlab.com/handbook/engineering/career-development/matrix/engineering/development/

/cc @sgoldstein @clefelhocz1

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