[MOD/FIX] Further HTML validating

[Denperidge]

In progress PR to finish up #95

Progress

• Dev-docs: fix headers
• Dev-docs: valid HTML (blocked by needed feedback, see below)
• Install: fix headers
• install: valid HTML

Needed feedback

Dev-docs

• Does anyone know what the restore_aria_label usage is in devs-docs?
• There is a <label for="tag_since"> without a matching input. I originally assumed it was just because it was a code example (as other code examples include <label for="tag_per_source">), but it doesn't show up on the page. Did some parsing go wrong there?

Note: After this PR, HTML validators might still error because of the lack of alt text on images. This is out of scope of this PR, as there is a general lack of alt text throughout the repo

[Denperidge]

Outline comparison below for my own satisfaction, sorry!

Before changes	After changes

[Denperidge]

Hi hi! Rebased the master branch changes into mine and did a double check. Mentioning @Baraka24 for the remaining questions!

• in the documentation page, an <a> element has an incorrect type attribute (which, while existing, refers to mime types instead of the input type="button"). Is this intentional or can it be fixed?
• Both points in the original PR description (restore_aria_label & the stray label element) are still a problem. Any news on whether they are meant to be that way?

[GedeonTS]

in the documentation page, an element has an incorrect type attribute (which, while existing, refers to mime types instead of the input type="button"). Is this intentional or can it be fixed?

@christer77 could you take a look? It looks like this anchor was introduced in this commit