Standardize fenced code block formatting and add missing language tags

[hritikkumar07]

This PR resolves issue #976 by standardizing Markdown fenced code block formatting, correcting nesting syntax, and adding missing language identifiers to improve documentation readability and syntax highlighting consistency.

Changes Made

1. R Guide (Rguide.md):

   • Added the missing r language identifiers to 5 fenced code blocks to enable correct syntax highlighting for R code examples.

2. Style Guide (docguide/style.md):

   • Corrected nested fenced code block formatting in the "Nest codeblocks within lists" section. Used 4 backticks for the outer Markdown code block so that the nested 3-backtick C++ code block doesn't prematurely terminate it and break the parser.

3. Objective-C Guide (objcguide.md):

   • Standardized 4 code blocks that were using the objc tag to use objectivec instead, ensuring consistency with the other 98 Objective-C code blocks in the document.

Verification

• Audited all Markdown files in the repository using a custom Node.js Markdown parser to verify that all code blocks are cleanly formatted and contain standard, supported language identifiers.

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[vapier]

objc is valid

gh-pages uses Rouge by default for syntax highlighting:
https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll#syntax-highlighting

the "correct" name is objective_c, not objectivec. but objectivec & objc are aliases of objective_c. so i see no reason to change this.

https://github.com/rouge-ruby/rouge/blob/16e6ecdb3bc4248cead78375e1b24580ee2352a1/lib/rouge/lexers/objective_c.rb#L15

[vapier]

I don't see the point of this

[vapier]

feel free to re-open if you make more changes