Add completion command to commands list in RTD

[juhoinkinen]

When the annif completion command was added in #693, it was forgotten to add the command in the CLI commands page that is shown in ReadTheDocs (updating that page is only semi-automatic).

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fd8561d) 99.67% compared to head (4bcbf28) 99.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #732 +/- ## ======================================= Coverage 99.67% 99.67% ======================================= Files 89 89 Lines 6405 6405 ======================================= Hits 6384 6384 Misses 21 21 ``` | [Files Changed](https://app.codecov.io/gh/NatLibFi/Annif/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NatLibFi) | Coverage Δ | | |---|---|---| | [annif/cli.py](https://app.codecov.io/gh/NatLibFi/Annif/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NatLibFi#diff-YW5uaWYvY2xpLnB5) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.