Issue#1006: Minor documentation fixes

[JHertz5]

Description Resolves #1006. Warnings that I've corrected:

• Trailing whitespace in docs/rule_groups/structure_optional_rule_group.rst. (This isn't a warning raised by Sphinx, but I introduced it without realising and would like to correct it).
• code-block directive called in docs/editor_integration/vim.rst, rather than the actual directive code_block. This leads to the code blocks on this page not being rendered.
• Underline lengths in docs/rule_groups/whitespace_rule_group.rst
• Undefined label configuring-conditional-expressions-structure-rules. This link is not used anywhere, so I've deleted it.
• No title or caption to be referenced for configuring-simple-multiline-structure-rules. Moved title to the top for the reference anchor on to more easily.

Warnings that I've ignored:

• configuring_selected_assignment_structure_rules.rst missing from docs/configuring.rst. (It looks like this page looks incomplete and claims to describe selected_assignment_001, but describes a rule that is much more complex than selected_assignment's current functionality.
• notepad_pp.rst missing from docs/editor_integration.rst. (This page looks incomplete).
• Several duplicate label warnings. (They're not a problem as long as we don't reference the labels, I think).
• api.rst, release_process.rst, and rule_selection.rst missing not included in any toctrees. (I'm not sure where they would go. It looks like they might be not be "customer facing" and therefore are deliberately not linked).
• No pygments lexer for viml. (Nothing we can do about that).

Screenshots None

Additional context None

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (92d966d) 95.87% compared to head (e943fd5) 95.87%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1007 +/- ## ======================================= Coverage 95.87% 95.87% ======================================= Files 1500 1500 Lines 28262 28262 ======================================= Hits 27095 27095 Misses 1167 1167 ```

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[jeremiah-c-leary]

Afternoon @JHertz5 ,

Thanks for the documentation updates. I typically do not build from scratch so I tend to miss these issues.

I will merge this to master.

--Jeremy