Improvements to the Install DXP page

[mnocon]

Target: master, 4.6

Things done:

1. Fixed missing space and wrong usage of eventually. It's a trap that I fall into quite easily, so I started noticing it :D See https://quicksilvertranslate.com/243/false-friends-eventually/
2. Removed things that are no longer relevant:

• search in the frontend is no longer needed, since we removed the Old Commerce solution (but it's still strongly recommended)
• JMS secret is no longer required - it's gone with the Old Commerce
• the .gitignore file is already present in recent releases: https://github.com/ibexa/experience-skeleton/blob/v4.6.7/.gitignore

3. We need to send people to Composer doc to install it, because checking the checksum is a security recommendation and we should not advise against it
4. I want to change the header name for setting up the HTTP server - right now it mentions production, but the very first instruction is for a development setup.
5. Fixed list formatting in the Elasticsearch article

Previews:
Old: https://doc.ibexa.co/en/latest/getting_started/install_ibexa_dxp/
New: https://ez-systems-developer-documentation--2422.com.readthedocs.build/en/2422/getting_started/install_ibexa_dxp/

Old: https://doc.ibexa.co/en/master/search/search_engines/elastic_search/configure_elastic_search/
New:https://ez-systems-developer-documentation--2422.com.readthedocs.build/en/2422/search/search_engines/elastic_search/configure_elastic_search/

[juskora]

@mnocon I know it's not the scope but maybe with this PR we could also get rid of collapsible section in line 134 as it brought some problems recently.

[mnocon]

@mnocon I know it's not the scope but maybe with this PR we could also get rid of collapsible section in line 134 as it brought some problems recently.

We are switching the default PHP version to 8.3 in the upcoming release (next week) and we will have to adjust this section anyway - that's why I didn't touch it in this PR (I agree that we should improve this)