Docs: Add google tag manager

[louismaximepiton]

Description

• Changed the .gitmodules url in order to not force people to fork ods-jekyll-theme as well.
• Added the cookies management using TarteAuCitron
• Added the custom scripts and custom CSS
• Added a page in the doc to handle the cookies once the first interaction is made.
• Generated the hash using https://www.srihash.org/

What needs to be reviewed at least

• The Cookies panel is visible (make sure to not use Firefox)
• Make sure the cookies are fine
• The Panel can be accessed from somewhere inside the website
• Define what to put inside the Cookies page
• Make sure the changes in main.html aren't breaking

Misc

Need to be careful once the ods-jekyll-theme is updated with boosted v5.3.x, both the script and the associated css need to be changed.
Should we try to set it on ods-jekyll-theme lib to ease the maintenance on all lib using the theme ?

[netlify]

✅ Deploy Preview for ods-android canceled.

Name	Link
🔨 Latest commit	6a55956
🔍 Latest deploy log	https://app.netlify.com/sites/ods-android/deploys/65ccd02b95f3ab000865859a

[paulinea]

@louismaximepiton Thank you for this submission, it works. I think it's a good proposition to add it in the ods-jekyll-theme. It can be done in a second time.

[julien-deramond]

LGTM! Good job @louismaximepiton
Let's try to tackle my comment to help future contributors build a local env.
After the merge of this PR, we'll need to double-check that:

• analytics are available in the tool
• when pages are embedded to Zeroheight, the cookies banner is not displayed (and no cookies are stored obviously)

I'll then create an issue in https://github.com/Orange-OpenSource/ods-jekyll-theme to try to embed these modifications directly into the theme.