WIP: DOC: Add Technical Reference page for reference and anchor points

[yvonnefroehlich]

Description of proposed changes

Add a Technical Reference page for reference and anchor points for plot embellishments.

Related to #4028, #4027

Previews:

• https://pygmt-dev--4052.org.readthedocs.build/en/4052/techref/index.html
• https://pygmt-dev--4052.org.readthedocs.build/en/4052/techref/reference_anchor_points.html
• https://pygmt-dev--4052.org.readthedocs.build/en/4052/techref/justification_codes.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

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• /format: automatically format and lint the code

[seisman]

Please see changes in #4025. I think we should provide more details about the positioning of embellishments. The reference point is controlled by the position/position_type parameters, and the anchor point is defined by the anchor/anchor_offset parameters.

[yvonnefroehlich]

Totally agree @seisman! Will wait and leave this PR as draft until you have finished the work on this.

[seisman]

I think we should finish this PR first, so that we can reference this documentation in PRs like #4025.

[yvonnefroehlich]

Ah OK, that's also fine with me. Hope I will have time to work on this PR in the next days.