docs: Trim down spelling list

We have accumulated a number of words in the spelling list over time,
and some of them are no longer necessary (especially now that we ignore
Helm value names from the Helm reference). Let's trim down the list.

The following process was applied:

    1. Empty the list of spelling exceptions
    2. Run 'make html' to get the command to add missing words
       (update-spelling_wordlist.sh)
    3. Run that command to add all words
    4. Remove all entries starting with an upper case from the list, as
       some of them are unnecessary given that they have a corresponding
       lowercase entry (which is case-insensitive)
    5. Re-run 'make html' to get the command to add the exceptions that
       are still missing
    6. Run that command
    7. Manually edit the file to make sure we don't change/add existing
       entries (no Git addition)
    8. Re-add names of contributors that are recognised locally, but
       that CI fails to recognise because it doesn't have the Git
       history

Signed-off-by: Quentin Monnet <quentin@isovalent.com>