Fix mermaid flowchart syntax in Indexables technical specification

[schlessera]

There are two syntax issues with the first mermaid diagram - a mix-up between == and -- syntax within a single directional relation.

I assume this worked before but now broke because the validation in the mermaid parser became stricter.

Given that I don't know what purpose the differentiation between --> and ==> was supposed to signify (and it does not come with any legend to this regard), I opted to just strip all ==> (bold) relations and consistently use --> throughout the diagram.

Summary

Replaces all the different types of arrows with a consistent --> (single-dash) syntax.

Relevant technical choices

Technical choices that affect more than this issue:

Test instructions

This PR can be acceptance tested by following these steps:

1. First diagram should render correctly at https://developer.yoast.com/features/indexables/technical-specification/

Quality assurance

• Security - I have thought about any security implications this code might add.
• Performance - I have checked that this code doesn't impact performance (greatly).
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• Tested - I have tested this code to the best of my abilities.
• Automated tests - I have added unit tests to verify the code works as intended.
• Testability - I have added unique ids to elements, so they can be located in automated testing.
• I have altered a filename.
  • I have adjusted the ID property accordingly and updated all internal links.
  • I have added the redirect to the _redirects file in the root of the project.

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