Ugrade instructions from 2.6.x to 2.6.y (git clone)

[Krysztophe]

• There is no documention for upgrading between two 2.6 releases in https://doc.wallabag.org/admin/upgrade/

• This worked for me with from 2.6.10 to 2.6.12 (on Debian 12, installation from git clone)

git pull
git checkout 2.6.12
bin/console cache:clear --env=prod     #  to avoid an error 500 

• I do not understand when I have to run (or not) make install
  (in fact I never had composer installed, although it seems compulsory?!)

• It seems that general instructions are hidden there in the migration to 2.3: https://doc.wallabag.org/admin/upgrade/#upgrade-on-a-shared-hosting
  This may need a copy-paste from elsewhere or a pointer?

• If some of this is correct, I can try to make a PR on the doc

[j0k3r]

Maybe a rewrite/cleanup is necessary