chore: Don't overwrite _disabledToc when metadata is explicitly defined

[filzrev]

This PR fix problem occurred by #9737.

What's changed in this PR

• Set _disableToc metadata only when _disableToc is not defined yet.
• Set _disableToc to index.md file to suppress HTML snapshot diffs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.99%. Comparing base (fe673ec) to head (fe56b5a).
Report is 51 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9741      +/-   ##
==========================================
+ Coverage   74.31%   78.99%   +4.67%     
==========================================
  Files         536      536              
  Lines       23189    23194       +5     
  Branches     4056     4027      -29     
==========================================
+ Hits        17234    18323    +1089     
+ Misses       4853     3748    -1105     
- Partials     1102     1123      +21     

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[filzrev]

Screenshot files for snapshot tests are updated by PR (#9744)
I'll update index.html.verified.html file after above PR is merged.

[filzrev]

I've added commit(9664a7b) that remove _disableToc metadata from seed site.
Because it specify _layout: landing so _disableToc settings is not works anyway.

_disableToc: false setting is expected to be used for backward compatibility for sites that don't used _layout: landing.

I've manually confirmed _disableToc: false setting works as expected with following steps.

1. Create docfx site with docfx init --yes command.
2. Remove _layout: landing setting from index.md.
3. Run docfx build --serve and confirms TOC is not displayed.
4. Add _disableToc: false setting to index.md.
5. Run docfx build --serve and confirms TOC is displayed.