docs: expands the client_libraries page and adds a glossary

[JavierMtzRdz]

Summary:

This PR expands the client_libraries page and adds a glossary, superseding the corresponding changes made in #1697.

• Added a Glossary page that redirects to the external Google Doc (based on internal glossary).
• Relocated "API Clients" to the top-level API documentation (redirects from covidcast clients page are now automatic).
• Updated the content of "API Clients" based on the "Getting Started" vignettes.
• Updated the link to the API Keys page.

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

[nmdefries]

The content and organization is great! Most of my comments are little wording changes. Once those are incorporated (or otherwise handled, if you disagree/want to discuss) and we check that this still renders properly, this can be merged.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[JavierMtzRdz]

Suggestions and issues discussed so far have been integrated.