Consistenize Documentation links

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

(follow-up to #12582 and #12623)

Currently the Documentation links are all over the place, this is a tracker / reminder issue to fix this, currently the link types used are:

• dot relative: ./api/somewhere
• non-dot relative: api/somewhere
• no-prefix absolute: /docs/api/somewhere (mostly fixed by #12623)
• prefix absolute: https://mongoosejs.com/docs/api/somewhere

Note: the above names are not official ones, i just used what comes to mind for me

Anything that is absolute would not work well with versioning (as pointed out by the mentioned issue and PR) and not using a consistent style is not too easy to read.

I would also propose to add some note to CONTRIBUTING#documentation.

Also there would be the question of changing the API documentation (jsdoc) links to be consistent, but i am not sure on what we should change it to: relative or prefix-absolute, relative would work well with documentation and versioning, but would be hard to access through jsdoc (though this could be fixed by tsdoc in .d.ts) and could also be fixed by post-processing links in the api generation to be relative.

[hasezoey]

Also, I ran the https://mongoosejs.com/ under a broken links checker:

may need to be re-run after #12623 and #12644

[hasezoey]

I guess the conclusion from #12623 is that the following link-style will be used: “no prefix relative paths”: guide.html instead of something like ./guide.html and only when a prefix is needed like ../ for ../guide.html from something like tutorials/index.html