fix: give relative paths explicit .md suffix so they work in GitHub web UI too [ciskip]

[jessicaschilling]

Well, this was painful. 😭

This PR replaces html-style relative paths throughout the entirety of the docs with ones that are "explicit" in two ways:

• have correct use of slashes, dots etc at the beginning to properly traverse directory structure, rather than relying on Vuepress kindly making its usual assumptions about relative paths
• include .md as appropriate in the actual file name

This enables people who are reading the docs in GitHub's web UI to actually have relative links that work. (VuePress is clever enough to strip out .md etc on site build so everything works correctly on docs.ipfs.io.)

Example:

[`ipfs pin -r`](/reference/cli/#ipfs-pin)

becomes

[`ipfs pin -r`](./reference/cli.md#ipfs-pin)

As this was an entirely manual process, I may have missed a few. However, have checked all modifications in this PR both on a local site build and on GitHub web UI.

Also adds guidance for docs writers on the "Grammar, formatting, and style" page on adding relative paths (example).

ref #669 (comment) from @kvutien.

[cwaring]

💯🙏🏻