[feat]: Support for global bibliographies

[thdhondt]

Hello @shyamd!

One feature that would be very useful for my work would be a global and latex-like bibliography. While thinking about this feature, I noted the following:

• The content of the global bibliography should be independent of where the \full_bibliography tag is located in the documentation.
• Footnotes are well suited for local bibliography, but not for a global one as they can't really refer to another page.
• The indices used to refer to a bibliography entry should be unique across the whole documentation for a global bibliography

I gave this project a try and ended up doing a pretty extensive rewrite of the plugin ...
I tried to fix the first issue by pre-parsing the files of the documentation in the on_file mkdocs callback.
The second issue is handled by introducing different citation formats (footnote, inline, link).
The last issue is fixed by having global or per-page bibliography indexes depending on the setting chosen by the user.

I have also changed some of the configuration options of the plugin.
The default behavior results in the same output as today:

plugins:
  - bibtex:
      bib_file: "refs.bib"
      cite_format: "footnote"
      bib_type: "per_page"

A global bibliography would be configured this way:

plugins:
  - bibtex:
      bib_file: "refs.bib"
      cite_format: "link"
      bib_type: "global"
      global_bib_ref: "\bibliography.md"

Even though there are quite a lot of changes that should still be made before this can be merged, it is in a mostly functional state. I was curious if you would be interested in taking over some of these changes in the official plugin.

I have not yet updated any of the tests, documentation, etc.