Add js string codebytes

[mehboobali98]

Description

This closes (#2513)
Added codebyte examples to string .charAt(), .concat() and .indexOf() methods.

Type of Change

• Adding a new entry

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• Under "Development" on the right, I have linked any issues that are relevant to this PR (write "Closes # in the "Description" above).

[SSwiniarski]

@mehboobali98 I did a first review and left a couple of comments.

[mehboobali98]

@SSwiniarski Resolved the comments.

[SSwiniarski]

@mehboobali98 looks good for a second review.

[caupolicandiaz]

@mehboobali98 @SSwiniarski I've added a few additional edits to these entries. Looks good to go.

[github-actions]

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[SSwiniarski]

@mehboobali98 congrats on the new edits!

Live links:

• https://www.codecademy.com/resources/docs/javascript/strings/charAt
• https://www.codecademy.com/resources/docs/javascript/strings/concat
• https://www.codecademy.com/resources/docs/javascript/strings/indexOf