Add API documentation - Githubissues

Changelogs

Add API section and corresponding md files, update mkdocs.yml.

Checklist:

[ ] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
[ ] Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
[x] Update the API documentation if a new function is added, or an existing one is deleted.
[x] Write concise and explanatory changelogs above.
[x] If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

discussion related to that PR