Various improvements to docs and code

[giovannipizzi]

First a number of sections have been added to the docs:

• clean_storage is now mentioned in the basic usage
• we also add basic examples of the new CompressMode.AUTO functionality
• Maintenance operations are documented and explained
• We discuss the long-term support of data even without this library (fixes #100)
• Links to the original performance tests
• Document the new compression modes
• Explain that nowadays clean_storage is safe to use concurrently on all platforms
• Explain how to reclaim space with repacking of packs after deletion

In addition, some changes to the code have been performed:

• utf8 is explicitly specified for the config.json file
• get_hash is renamed to get_hash_cls (and get_hash is deprecated`) -

Some of these changes address the remaining comments and thus fix #101, as well as the remaining comments and thus fix #3.

Extending the documentation

[giovannipizzi]

Note: ideally this should be merged after #148 (and rebased upon it) as the docs describe features implemented in that PR

[zhubonan]

Note: ideally this should be merged after https://github.com/aiidateam/disk-objectstore/pull/148 (and rebased upon it) as the docs describe features implemented in that PR

Yeah I agree, better to have #148 merged first.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (afdae26) 99.94% compared to head (0e5789f) 99.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #151 +/- ## ======================================= Coverage 99.94% 99.94% ======================================= Files 8 8 Lines 1858 1862 +4 ======================================= + Hits 1857 1861 +4 Misses 1 1 ``` | [Impacted Files](https://app.codecov.io/gh/aiidateam/disk-objectstore/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | Coverage Δ | | |---|---|---| | [disk\_objectstore/container.py](https://app.codecov.io/gh/aiidateam/disk-objectstore/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam#diff-ZGlza19vYmplY3RzdG9yZS9jb250YWluZXIucHk=) | `99.88% <100.00%> (ø)` | | | [disk\_objectstore/utils.py](https://app.codecov.io/gh/aiidateam/disk-objectstore/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam#diff-ZGlza19vYmplY3RzdG9yZS91dGlscy5weQ==) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[giovannipizzi]

Because of the merge conflict, this needs to be reapproved @zhubonan (Then I will squash-merge)