Various non-normative HTML fixes

[dr-shorthair]

fixes #143

[dr-shorthair]

Could one more person review so this can be merged?

[ldesousa]

@dr-shorthair This is a very large set of changes, since it is non-normative I didin't look into detail to the content, but a few points are worth considering:

1. There are many "... TODO ..." notes across the document. Is it on purpose? Eve if yes consider instead to open an issue pointing to the missing content.

2. It looks like some of the inner blue tables are missing cells (i.e. the background is not applied). Some examples below.

3. There are many broken links throughout the document. In Brave (Chromium) the HTML render marks those with a red wavy underline. A few examples below.

[dr-shorthair]

Thank you @ldesousa

1. The TODO comments are either

• need to redraw figures - see Re-draw figures - add new classes and properties #140 - but note that this will be deferred until the ontology is stable
• need for examples for the Collection terms - see new issue Add text examples for new classes #159

2. The inner blue tables show the axioms in the SSN graph. If you click the link [Hide additional SSN axioms] it will be hidden. I've added a note in the Modularization section drawing attention to this capability
3. I've fixed 7 broken links (internal cross-references). The red underlines associated with the show/hide buttons are an artefact of javascript and relative paths which should go away when the branch is merged.

I also accepted the lint corrections from @sgrellet

See https://raw.githack.com/w3c/sdw-sosa-ssn/fix-html/ssn/index.html#Specification for the revised state