Fix broken links on the website

[anermina]

Closes #792 .

There are two links I am not sure about and that weren't replaced:

1. link:/author/topics/document-format/meta-attributes#doc-history-misc-container
   Is the new link https://www.metanorma.org/author/ref/document-attributes#generic-metadata?

Context:

=== Document history

Semantic markup of document history can be added to the document,
link:/author/topics/document-format/meta-attributes#doc-history-misc-container[using Metanorma extension]
and Relaton YAML [added in https://github.com/metanorma/metanorma-ogc/releases/tag/v1.2.0].

2. link:/author/topics/document-format/sections#user-css
   It should be https://www.metanorma.org/author/topics/inline_markup/text_formatting#css?

[ronaldtse]

@anermina this is an interesting topic because it feels that some information was lost:

• https://github.com/search?q=org%3Ametanorma%20doc-history-misc-container&type=code

There are 4 entries that refer to this "semantic document history" location but it doesn't exist.