Add new FrontmatterTitle Vale rule and fix errors raised by new rule

Why is this change being made?

From https://gitlab.com/gitlab-com/content-sites/docsy-gitlab/-/blob/main/content/docs/frontmatter.md, we need every content page to have its level 1 header declared in frontmatter.

This Vale rule ensures that every page follows this frontmatter rule.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to a code owner, typically a handbook maintainer
  • If you're unsure, please mention the handbook triager
• For transparency, share this MR with the audience that will be impacted
  • #handbook Slack channel if there is any user impact
  • #mr-buddies Slack channel if changes may impact more users
  • #whats-happening-at-gitlab linking to this MR if a large number of team members are impacted

Commits

%{all_commits}

---

.gitlab/CODEOWNERS

@@ -503,7 +503,6 @@
 /content/handbook/resellers/training.md @akramer @ecepulis
 /content/handbook/sales/ @akramer @jakebielecki
 /content/handbook/sales/account-planning/ @lschoenfeld @akramer @emelier
-/content/handbook/sales/assistance-from-investors.md @jakebielecki
 /content/handbook/sales/build-value-with-customers.md @akramer
 /content/handbook/sales/channel/ @ecepulis
 /content/handbook/sales/club/ @kdemarest @kagarrett