Revamp predefined variables page

What does this MR do?

A revamp to the Predefined variables page.

• The largest change is that the variables for merge request pipelines and external pull request pipelines are pulled out into their own sub-table.

  By doing this we can eliminate all the repetition that say things like Only available if [the pipelines are for merge requests](../merge_request_pipelines/index.md) and the merge request is created, and instead put that information in the intro to the table. This keeps the tables smaller and more manageable, and dramatically reduces the word/character count.

  It also makes it easier to see that these variables are only available in those specific pipelines.

• Aligns the language around the words present, included and available. I decided to standardize on available, in the sense of "the variable is 'available' in the job in case X".

• Aligns most of the grammar so that the same structure and grammar is used throughout. The structure being essentially:

  [Simple description][Availability][default value or example]

  Every variable had a different mix of these 3 bits of information, but they are now always ordered in the same way.

Related issues

Resolves #300722 (closed)

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