Make all headings subheadings of the title of the page.

[robinlinden]

This is more accessible, semantically correct, and better for SEO. See: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Heading_Elements#Usage_notes

We should also in the future make sure that .md documents with a title specified in the front-matter start their headings at ##.

After discussion on IRC, we will now automatically increment headings by 1 instead of requiring the contributor to fix their markdown.

---

This change is 

[iphydf]

---

Reviewed 9 of 9 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[iphydf]

Reviewed 1 of 1 files at r2. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[iphydf]

Reviewed 1 of 11 files at r3. Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[cebe]

Reviewed 1 of 11 files at r3. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[cebe]

---

Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[iphydf]

@zetok this is missing your LGTM. @robinlinden can you rebase this?

---

Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[zetok]

---

Reviewed 1 of 11 files at r3. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[fcore117]

Reviewed 1 of 11 files at r3. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

---

Comments from Reviewable