[DOCS]: Add "Forms" page to branch 7.0

Description

We need to move the content from legacy repo and convert it to RST, as well as update the content.

[!IMPORTANT]
Please read the below resources before contributing to help you with this task:

• Contributing guidelines
• Contributing to Mautic’s documentation
• Mautic technical writing style guide
• The "Suggestions (Optional)" section below for guides on how to update the page

Relevant Documentation URL

https://devdocs.mautic.org/en/7.0/plugin_miscellaneous/forms.html

Link to the content at legacy docs

https://github.com/mautic/developer-documentation/blob/main/source/includes/_plugin_misc_forms.md

Are you willing to help us fix this by making a pull request?

No

Suggestions (Optional)

How to update the page

• Find the file that needs to be updated in the docs/ directory. The dev docs URL format is as such:

  https://devdocs.mautic.org/en/x.x/FOLDER_NAME/FILE_NAME.html
  

  For example, if the relevant URL is https://devdocs.mautic.org/en/7.0/plugin_extensions/manipulating_contacts.html, then find manipulating_contacts.rst file inside the plugin_extensions folder.

• Transfer the content under the note admonition. Look at other pages on how to structure the content.

• Review and update the content when you see fit.

• Follow the contributing guidelines that are listed in the "Resources" section above.

Screenshots or Screen Recordings (Optional)

No response

Start with the legacy content in source/includes/_plugin_misc_forms.md and the target page path implied by the URL: docs/plugin_miscellaneous/forms.rst. Read the contributing guidelines and nearby RST pages for structure and style, then transfer and update the Forms content. Done means the page is present in branch 7.0, uses valid RST, and reflects the reviewed legacy content.