decision-process: Use headers, fix some links, some rearranging
This commit is a follow-up to PR #105. My changes are summarized below:
* Move decision-making page to `policy/` subfolder
* Easier to determine docs that require full consensus for changes
* Fix link to D&I team members by linking out to FAS group members
* Change formatting of some sections to headers
* Add "Choosing a process" section from previous section
* Made some edits for clarity / improved readability
Signed-off-by: Justin W. Flory <git@jwf.io>