Add help for the chart (fixes DEV-156)

[aptiko]

Checklist:

• All tests are passing
• I have added any necessary documentation and/or release notes
• I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• I have removed all obsoleted code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (9531560) to head (4a6d05f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
- Coverage   97.48%   97.47%   -0.02%     
==========================================
  Files          55       55              
  Lines        3543     3528      -15     
==========================================
- Hits         3454     3439      -15     
  Misses         89       89              

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[aptiko]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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[Copilot]

Pull request overview

This pull request adds an interactive help/information button to the chart interface that displays an annotated example chart in a modal dialog, explaining how the chart visualization works. This addresses ticket DEV-156 by moving chart documentation from a standalone documentation file into inline, user-accessible help.

Changes:

• Added an info button (ⓘ) to the chart toolbar that opens a modal with an annotated chart example
• Removed standalone chart documentation (doc/general/chart.rst) in favor of inline help
• Added Greek translations for the new help content

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
enhydris/templates/enhydris/timeseries_group_detail/chart.html	Added Bootstrap modal with SVG overlay annotations explaining chart behavior
enhydris/static/js/enhydris.js	Added custom toolbar icon that triggers the info modal
enhydris/static/css/enhydris.css	Added styles for chart legend modal, refined z-index placement, and SVG overlay positioning
enhydris/templates/enhydris/timeseries_group_detail/main-default.html	Added translation string for the info button tooltip
enhydris/locale/el/LC_MESSAGES/django.po	Added Greek translations for chart help text and button label
enhydris/locale/el/LC_MESSAGES/django.mo	Compiled translation file
enhydris/static/images/chart-legend.png	Added example chart image for the help modal
doc/index.rst	Removed reference to deleted chart documentation
doc/general/chart.rst	Removed standalone chart documentation file

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