Clean up docs warnings detected by the Vale spelling rule

Problem to solve

The vale spelling rule currently gives 931 warnings. We should be able to reduce this number a lot, with the following solutions:

• Expand the spelling exceptions list (https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/spelling-exceptions.txt)
• Fix typos
• Put certain things in backticks

Rule: https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/Spelling.yml