Add cypress tests

[matevz]

Add cypress tests which spin up the documentation site locally and:

• [ ] visit landing page, reloading, clicking on different kind of links (sidebar, other internal page, external page, top-left logo)
• [ ] visit some inner general, oasis-core, oasis-sdk, oasis-core-ledger pages
• [ ] check generated pages (type: autogenerated)
• [ ] check specific components like DocCard (both generated and manual)
• [ ] checking no-javascript version of the site

All tests should click and check, if the links on the apge are correctly working and then also try reloading the site and checking them again (e.g. to catch #4).

[lukaw3d]

All tests should click and check, if the links on the apge are correctly working and then also try reloading the site and checking them again (e.g. to catch https://github.com/oasisprotocol/docs/issues/4).

Locally you also need to append / to URL before reloading. When reloading on github pages, it redirects to append /