Improve Requirements Management docs

What does this MR do?

Avariety of improvements:

• Change the empty state message for the Requirements page:

  • Make the sentence more focused on what the user can accomplish
  • Remove alliteration ("create criteria", or even "can create criteria")

  Before	After

• Change the page h1 from Requirements to Requirements Management to improve SEO, given that we have multiple other pages and sections called "Requirements", usually listing installation prerequisites.

• Improve the introduction, using information from the parent epic and direction page.

  Before:

  Requirements allow you to create criteria to check your products against. They
  can be based on users, stakeholders, system, software, or anything else you
  find important to capture.

  After:

  With requirements, you can set criteria to check your products against. They can be based on users,
  stakeholders, system, software, or anything else you find important to capture.
  
  A requirement is an artifact in GitLab which describes the specific behavior of your product.
  Requirements are long-lived. They don't disappear when it's completed.
  
  If an industry standard *requires* that your application has a certain feature or behavior, you can
  [create a requirement](#create-a-requirement) to reflect this.
  When a feature is no longer necessary, you can [archive the related requirement](#archive-a-requirement).

• Remove an unnecessary image and line ("The requirements list shows the new title immediately.")

Related issues

#212889 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• Apply the documentation label, plus:
  • The corresponding DevOps stage and group label, if applicable.
  • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
  • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
  • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
• Assign the designated Technical Writer.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.