without this click includes the full (long) docstring in the main --help

Closes T2669

Closes T2699.

h/t @haltode

also apply various other minor improvements to the configuration documentation:

- mention and show in the example the outer "swh > fuse" stanza
- fix sphinx markup for verbatim text (it's "``", not "`")
- add configuration file doc to the doc index

- Closes T2705: move --config-file to "swh fuse".
- Closes T2678: add default configuration location.

Closes T2665.

in particular:
- drop the TODO about separate cache DBs (now done)
- drop the examples part (we will have a tutorial/walkthrough)
- add reference to task dedicated to sharding

Do not unnecessarily prefetch information before accessing it directly.

as the alternative switch to --foreground

Fixes T2698.

Closes T2664.

- API data generation script can now easily prefetch content raw data,
  revision parents, etc.
- Use the Rust compiler as testing repository (contains more corner
  case, such as submodules)
- Mount test artifacts on the fly

Move the lookup() code logic into FuseEntry class.

Closes T2663.

- Add a `SymlinkEntry` class (+ rework fs/ class init using dataclasses)
- Support symlinks, submodules, and mounting revisions
- Basic unit tests for revision artifacts

Closes T2673.

We do not know the encoding beforehand so manipulate every blob coming
from the archive as bytes.

- Change class interface to match async interface (eg: iter -> aiter)
- Sync code is now running with `run_in_executor`
- Use aiosqlite as the async interface for sqlite

This is necessary to read very long files, or use a terminal pager.

See D4139.

Better class abstraction. Now the information is less scattered around
the code (especially for fuse.py). Each FUSE entry virtual type is
self-contained and defines entirely how it is used, listed, read, etc.

- Create a pytest fixture to run swh-fuse in a separate process and
  setup an offline mock web API
- Add basic cnt/dir tests

More flexible and easier to tweak settings. For example we can now have,
the metadata cache in-memory while the blob cache is on-disk:

  cache:
    metadata:
      in-memory: true
    blob:
      path: /cache/dir/blob.sqlite

Rework the cache architecture. The cache code is now separated into
multiple classes (one for each cache type, and one global). Every cache
is now used as a context manager and must implement `__getitem__` and
`__setitem__` to access/update the underlying cache information.