Plugins doc

[daks]

Hello,

I finally made very few changes on the doc to add information about plugins. Each one having its own doc, I only linked to the index page which contains the list of them.
Ref #776

[codecov-io]

Current coverage is 65.28%

Merging #813 into master will not affect coverage as of c5640ba

@@            master    #813   diff @@
======================================
  Files          112     112       
  Stmts         5660    5660       
  Branches         0       0       
  Methods                          
======================================
  Hit           3695    3695       
  Partial          0       0       
  Missed        1965    1965       

Review entire Coverage Diff as of c5640ba

---

Uncovered Suggestions

1. +0.44% via ...oa/lib/ldap_utils.py#65...89
2. +0.42% via ...ailbox_operations.py#65...88
3. +0.37% via ...commands/init.py#52...72
4. See 7 more...

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[tonioo]

Makes sense, thank you!