Better alert templates categorization (#1318)

* Update Doc to reflect recent AlertTemplates 

PMM-13273 - New MongoDB alert too_many_chunk_migrations by @sleto-it in #952
PMM-12726 - New PBM agent down alert by @sleto-it in #968
PMM-13382 - New PBM backup duration alert by @sleto-it in #970
PMM-12727 - New PBM backup size alert by @sleto-it in #969

* Doc for new MongoDB alert templates

* fix typo

* adds area / technology column in the alert table

* fix conflicts

* fix conflicts 2

* style

* new PBM area

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Co-authored-by: Roman Novikov <[email protected]>