plugins: Don't redundantly document common configurations

Description

1. Instead of redundantly documenting directory configuration option in each source plugin, refer to the built-in functionality documentation of the Source class.

2. For each element plugin derived from BuildElement, add reference to the built-in functionality documentation of BuildElement.

Changes proposed in this merge request:

• source.py: Add anchor for built-in functionality.
• sources/plugins/*.py: Remove directory config option, and add reference to Source's built-in functionality.
• buildelement.py: Add anchor for built-in functionality.
• plugins/elements/*.py: Add refence to BuildElement's built-in functionality for each build element.

This merge request, when approved, will close:

Fixes #713 (closed).

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