Rearrange the BookKeeper documentation, add decommissioning section. (#7230)
Rearrange the BookKeeper documentation, add decommissioning section. (#7230)

### Motivation

While discussing issue https://github.com/apache/pulsar/issues/7055#issuecomment-641038722 yesterday, @sijie suggested I add the documentation for decommissioning bookies to the Pulsar docs.

### Modifications

While adding the documentation for decommissioning, I also saw the BookKeeper sub sections didn't read well. There were duplicate sections for configuration as well.
So just rearranged them in a way that it would be relevant for work flow as well as added the decommissioning process details.

Rearranged the sections as:
- Hardware considerations
- Configure BookKeeper
- Deploy BookKeeper
- Starting bookies manually
- Decommissioning bookies cleanly
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