Updated 'The Picture element' section as per issue requirements

[a8h1km]

Why:

Closes: #35790

This change updates the "The Picture element" section to meet the issue requirements by simplifying the content and focusing on the <picture> HTML element support.

What's being changed (if available, include any code snippets, screenshots, or gifs):

• Updated the section previously titled "Specifying the theme an image is shown to" to "The Picture element."
• Replaced all content with the sentence: "The <picture> HTML element is supported."
• Removed outdated references to #gh-dark-mode-only and #gh-light-mode-only fragments.

Preview of the updated content:

# The Picture element

The `<picture>` HTML element is supported.

<!-- Let us know what you are changing. Share anything that could provide the most context.
If you made changes to the `content` directory, a table will populate in a comment below with links to the preview and current production articles. -->

### Check off the following:

- [ ] A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require a SME review from GitHub staff.
- [x] The changes in this PR meet [the docs fundamentals that are required for all content](http://docs.github.com/en/contributing/writing-for-github-docs/about-githubs-documentation-fundamentals).
- [x] All CI checks are passing.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

---

Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

---

fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@a8h1km Thanks so much for opening a PR with this fix! I've made one small edit to keep our title capitalization consistent 💛 I'll update the branch and get this merged once tests are passing 🍏

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡