Make consistent section heading levels

[hannahackermans]

Current State

Throughout cellproject's webpages, inconsistent heading levels are used and heading levels are skipped several times. To make the webpages more understandable, especially when using assistive technology, this needs to be consistent.
This is technically an advisory guideline in WCAG, but it needs to be solved anyway to meet the requirement stipulated in issue #60 , so I suggested we fix this first.

heading info per page

• https://cellproject.net/

Missing first level heading.
Also issues with the use of a layout table, described in issues #61

• https://cellproject.net/members

Missing first level heading.
The titles of the databases are not headings but made to look like headings.

• https://cellproject.net/manifesto

Missing first level heading.
Skipped h3 heading.
subsections do not have real headings, which leads to broken same-page links. See issue #60

• https://cellproject.net/search/site/

Missing first level heading.
Skipped h3 heading.
Also issues with the use of a layout table, described in issues #61

• https://cellproject.net/taxonomies-definition

Missing first level heading.
h2 is used for both the main title and the subsections.

Proposed changes to meet the success criteria

1. Make sure all pages use h1-hX without skipping heading levels
2. Make sure that text that visually looks like headings have a heading element.